Penicillin-binding protein expression at different growth stages determines penicillin efficacy in vitro and in vivo: an explanation for the inoculum effect

Recent development and fighting strategies for lincosamide antibiotic resistance

SUMMARYLincosamides constitute an important class of antibiotics used against a wide range of pathogens, including methicillin-resistant Staphylococcus aureus. However, due to the misuse of lincosamide and co-selection pressure, the resistance to lincosamide has become a serious concern. It is urgently needed to carefully understand the phenomenon and mechanism of lincosamide resistance to effectively prevent and control lincosamide resistance. To date, six mobile lincosamide resistance classes, including lnu, cfr, erm, vga, lsa, and sal, have been identified. These lincosamide resistance genes are frequently found on mobile genetic elements (MGEs), such as plasmids, transposons, integrative and conjugative elements, genomic islands, and prophages. Additionally, MGEs harbor the genes that confer resistance not only to antimicrobial agents of other classes but also to metals and biocides. The ultimate purpose of discovering and summarizing bacterial resistance is to prevent, control, and combat resistance effectively. This review highlights four promising strategies, including chemical modification of antibiotics, the development of antimicrobial peptides, the initiation of bacterial self-destruct program, and antimicrobial stewardship, to fight against resistance and safeguard global health.

Comparison of the inoculum effect of in vitro antibacterial activity of Imipenem/relebactam and Ceftazidime/avibactam against ESBL-, KPC- and AmpC-producing Escherichia coli and Klebsiella pneumoniae

OBJECTIVE

To evaluate effect of inoculum size of extended-spectrum β-Lactamase (ESBL)-producing-, AmpC-producing-, and KPC-producing Escherichia coli and Klebsiella pneumoniae on the in vitro antibacterial effects of imipenem/relebactam (IMR) and ceftazidime/avibactam (CZA).

METHODS

We compared the impact of inoculum size on IMR and CZA of sixteen clinical isolates and three standard isolates through antimicrobial susceptibility tests, time-kill assays and in vitro PK/PD studies.

RESULTS

When inoculum size increased from 105 to 107 CFU/mL, an inoculum effect was observed for 26.3% (5/19) and 52.6% (10/19) of IMR and CZA, respectively; time-kill assays revealed that the concentration of CZA increased from ≥ 4 × MIC to 16 × MIC to reach 99.9% killing rate against K. pneumoniae ATCC-BAA 1705 (KPC-2-, OXA-9- and SHV-182-producing) and 60,700 (SHV-27- and DHA-1-producing). While for IMR, a concentration from 1 × MIC to 4 × MIC killed 99.9% of the four strains. When the inoculum size increased to 109 CFU/mL, neither IMR nor CZA showed a detectable antibacterial effect, even at a high concentration. An in vitro PK/PD study revealed a clear bactericidal effect when IMR administered as 1.25 g q6h when inoculum size increased.

CONCLUSION

An inoculum effect on CZA was observed more frequent than that on IMR. Among the β-lactamase-producing strains, the inoculum effect was most common for SHV-producing and KPC-producing strains.

Should Linezolid Replace Clindamycin as the Adjunctive Antimicrobial of Choice in Group A Streptococcal Necrotizing Soft Tissue Infection and Toxic Shock Syndrome? A Focused Debate

Group A Streptococcus (GAS) necrotizing soft tissue infections and toxic shock syndrome remain high-mortality conditions. In vitro and animal model data, as well as multiple observational studies, suggest adjunctive clindamycin (ie, given with a beta-lactam) reduces invasive GAS infection mortality by inhibiting exotoxin production. Unfortunately, clindamycin resistance in GAS has been rapidly increasing in the United States since the mid-2010s, although the clinical significance of this remains unclear. Linezolid is a promising alternative adjunctive agent to which US GAS isolates remain near-universally susceptible, with a similar mechanism of action and similar in vitro evidence of GAS virulence factor attenuation. However, the clinical data supporting linezolid's value in severe GAS infections are far more limited. Here the authors review the data and reasoning behind a general preference for clindamycin or linezolid in a focused, pro-con debate format.

Antibiotic Disruption of the Gut Microbiota Enhances the Murine Hepatic Dysfunction Associated With a High-Salt Diet

Epidemiological and experimental evidence indicates that antibiotic exposure is related to metabolic malfunctions, such as obesity and non-alcoholic fatty liver disease (NAFLD). Liver impairment and hypertrophy of adipose cells are related to high salt consumption. This research aims to investigated the physiological mechanism of a high salt diet (HSD) enhanced antibiotic-induced hepatic injury and mitochondrial abnormalities in mice. The mice were fed a HSD with or without penicillin G (PEN) for 8 weeks and the gut metabolome, untargeted faecal metabolomics, and intestinal function were evaluated. The results revealed that HSD, PEN and their combination (HSPEN) significantly changed the gut microbial community. HSPEN mice exhibited more opportunistic pathogens (such as Klebsiella and Morganella) and reduced probiotic species (including Bifidobacterium and Lactobacillus). The main variations in the faecal metabolites of the HSPEN group were identified, including those connected with entero-hepatic circulation (including bile acids), tryptophan metabolism (i.e., indole derivatives) and lipid metabolism (e.g., erucic acid). Furthermore, increased intestinal permeability and immunologic response caused greater hepatic damage in the HSPEN group compared to the other groups. These findings may have important implications for public health.

Association between adjunct clindamycin and in-hospital mortality in patients with necrotizing soft tissue infection due to group A Streptococcus: a nationwide cohort study

Necrotizing soft tissue infection (NSTI) due to group A Streptococcus (GAS) is a severe life-threatening microbial infection. The administration of adjunct clindamycin has been recommended in the treatment of NSTIs due to GAS. However, robust evidence regarding the clinical benefits of adjunct clindamycin in NSTI patients remains controversial. We aimed to investigate the association between early administration of adjunct clindamycin and in-hospital mortality in patients with NSTI attributed to GAS. The present study was a nationwide retrospective cohort study, using the Japanese Diagnosis Procedure Combination inpatient database focusing on the period between 2010 and 2018. Data was extracted on patients diagnosed with NSTI due to GAS. We compared patients who were administered clindamycin on the day of admission (clindamycin group) with those who were not (control group). A propensity score overlap weighting method was adopted to adjust the unbalanced backgrounds. The primary endpoint was in-hospital mortality and survival at 90 days after admission. We identified 404 eligible patients during the study period. After adjustment, patients in the clindamycin group were not significantly associated with reduced in-hospital mortality (19.2% vs. 17.5%; odds ratio, 1.11; 95% confidence interval, 0.59-2.09; p = 0.74) or improved survival at 90 days after admission (hazard ratio, 0.92; 95% confidence interval, 0.51-1.68; p = 0.80). In this retrospective study, early adjunct clindamycin does not appear to improve survival. Therefore, the present study questions the benefits of clindamycin as an adjunct to broad spectrum antibiotics in patients with NSTI due to GAS.

Revisiting the inoculum effect for Streptococcus pyogenes with a hollow fibre infection model

Severe, invasive Streptococcus pyogenes (Strep A) infections result in greater than 500,000 deaths annually. First line treatment for such infections is benzylpenicillin, often with the addition of clindamycin, but treatment failure can occur with this regimen. This failure has been partially attributed to the inoculum effect, which presents as reduced antibiotic susceptibility during high bacterial density and plateau-phase growth. Hollow fibre infection models (HFIM) have been proposed as an in vitro alternative to in vivo research to study these effects. To re-evaluate the inoculum effect for benzylpenicillin, clindamycin, linezolid, and trimethoprim-sulfamethoxazole using a Strep A HFIM. Differential antibiotic susceptibility of Strep A was measured in a HFIM starting from low- and high-density inocula with an average difference in bacterial concentration of 56-fold. Dynamic antibiotic concentrations were delivered over 48 h to simulate in vivo human pharmacokinetics in an in vitro model. Differences in antibiotic susceptibility were measured by plate count of colony-forming units over time. Inoculum effects were seen in benzylpenicillin and linezolid at 24 h, and benzylpenicillin, linezolid, and clindamycin at 48 h. The effect size was greatest for continuously infused benzylpenicillin at 48 h with a log₁₀-fold difference of 4.02 between groups. No inoculum effect was seen in trimethoprim-sulfamethoxazole, with a maximal log₁₀-fold difference of 0.40. Inoculum effects were seen using benzylpenicillin, linezolid, and clindamycin, which may predict reduced clinical efficacy following treatment delay. The model has proven robust and largely in agreeance with published data, recommending it for further Strep A study.

Surgical Infection Society 2020 Updated Guidelines on the Management of Complicated Skin and Soft Tissue Infections

Background: The Surgical Infection Society (SIS) Guidelines for the treatment of complicated skin and soft tissue infections (SSTIs) were published in October 2009 in Surgical Infections. The purpose of this project was to provide a succinct update on the earlier guidelines based on an additional decade of data. Methods: We reviewed the previous guidelines eliminating bite wounds and diabetic foot infections including their associated references. Relevant articles on the topic of complicated SSTIs from 2008-2020 were reviewed and graded individually. Comparisons were then made between the old and the new graded recommendations with review of the older references by two authors when there was disparity between the grades. Results: The majority of new studies addressed antimicrobial options and duration of therapy particularly in complicated abscesses. There were fewer updated studies on diagnosis and specific operative interventions. Many of the topics addressed in the original guidelines had no new literature to evaluate. Conclusions: Most recommendations remain unchanged from the original guidelines with the exception of increased support for adjuvant antimicrobial therapy after drainage of complex abscess and increased data for the use of alternative antimicrobial agents.

In-hospital mortality among patients with invasive non-group A β-hemolytic Streptococcus treated with clindamycin combination therapy: a nationwide cohort study

Aim

Combination treatment with clindamycin is recommended in patients with invasive group A Streptococcus infection; however, whether the same treatment is effective in invasive group B Streptococcus and S. dysgalactiae subspecies equisimilis infections remains unknown. We aimed to investigate whether clindamycin added to standard of care therapy would be effective in patients with invasive non-group A β-hemolytic Streptococcus infections.

Methods

This was a nationwide retrospective cohort study using the Japanese Diagnosis Procedure Combination inpatient database focusing on the period between 2010 and 2018. We extracted data on patients diagnosed with sepsis due to non-group A β-hemolytic Streptococcus. One-to-four propensity score-matching was undertaken to compare patients who were treated with clindamycin within 2 days of admission (clindamycin group) and those who did not (control group). The primary outcome was in-hospital mortality.

Results

We identified 3754 eligible patients during the study period. The patients were divided into the clindamycin (n = 296) and control groups (n = 3458). After one-to-four propensity score matching, we compared 289 and 1156 patients with and without clindamycin, respectively. In-hospital mortality did not significantly differ between the two groups (9.7% versus 10.3%; risk difference 0.3%; 95% confidence interval, -3.5% to 4.2%).

Conclusions

This nationwide database study showed that combination therapy involving the use of clindamycin was not associated with lower in-hospital mortality in patients with invasive non-group A β-hemolytic Streptococcus.

Mathematical modeling of the 'inoculum effect': six applicable models and the MIC advancement point concept

Antimicrobial treatment regimens against bacterial pathogens are designed using the drug's minimum inhibitory concentration (MIC) measured at a bacterial density of 5.7 log₁₀(colony-forming units (CFU)/mL) in vitro. However, MIC changes with pathogen density, which varies among infectious diseases and during treatment. Incorporating this into treatment design requires realistic mathematical models of the relationships. We compared the MIC–density relationships for Gram-negative Escherichia coli and non-typhoidal Salmonella enterica subsp. enterica and Gram-positive Staphylococcus aureus and Streptococcus pneumonia (for n = 4 drug-susceptible strains per (sub)species and 1–8 log₁₀(CFU/mL) densities), for antimicrobial classes with bactericidal activity against the (sub)species: β-lactams (ceftriaxone and oxacillin), fluoroquinolones (ciprofloxacin), aminoglycosides (gentamicin), glycopeptides (vancomycin) and oxazolidinones (linezolid). Fitting six candidate mathematical models to the log₂(MIC) vs. log₁₀(CFU/mL) curves did not identify one model best capturing the relationships across the pathogen–antimicrobial combinations. Gompertz and logistic models (rather than a previously proposed Michaelis–Menten model) fitted best most often. Importantly, the bacterial density after which the MIC sharply increases (an MIC advancement-point density) and that density's intra-(sub)species range evidently depended on the antimicrobial mechanism of action. Capturing these dependencies for the disease–pathogen–antimicrobial combination could help determine the MICs for which bacterial densities are most informative for treatment regimen design.

Inoculum effect of β-lactam antibiotics

The phenomenon of attenuated antibacterial activity at inocula above those utilized for susceptibility testing is referred to as the inoculum effect. Although the inoculum effect has been reported for several decades, it is currently debatable whether the inoculum effect is clinically significant. The aim of the present review was to consolidate currently available evidence to summarize which β-lactam drug classes demonstrate an inoculum effect against specific bacterial pathogens. Review of the literature showed that the majority of studies that evaluated the inoculum effect of β-lactams were in vitro investigations of Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Across all five pathogens, cephalosporins consistently displayed observable inoculum effects in vitro, whereas carbapenems were less susceptible to an inoculum effect. A handful of animal studies were available that validated that the in vitro inoculum effect translates into attenuated pharmacodynamics of β-lactams in vivo. Only a few clinical investigations were available and suggested that an in vitro inoculum effect of cefazolin against MSSA may correspond to an increased likeliness of adverse clinical outcomes in patients receiving cefazolin for bacteraemia. The presence of β-lactamase enzymes was the primary mechanism responsible for an inoculum effect, but the observation of an inoculum effect in multiple pathogens lacking β-lactamase enzymes indicates that there are likely multiple mechanisms that may result in an inoculum effect. Further clinical studies are needed to better define whether interventions made in the clinic in response to organisms displaying an in vitro inoculum effect will optimize clinical outcomes.

Emerging erythromycin and clindamycin resistance in group A streptococci: Efficacy of linezolid and tedizolid in experimental necrotizing infection

OBJECTIVES

Clindamycin (CLI) and erythromycin (ERY) resistance is increasing among group A streptococci (GAS) causing invasive disease and alternative treatments are urgently required. In this study, the efficacy of the newer oxazolidinone tedizolid (TZD) was compared with the first drug in this class, linezolid (LNZ), in experimental murine myonecrosis caused by ERY-susceptible/CLI-susceptible (ERY^S/CLI^S) or ERY- resistant/CLI-resistant (ERY^R/CLI^R) GAS.

METHODS

Normal adult outbred Swiss Webster female mice (10 per group) were infected intramuscularly with ERY^S/CLI^S (ATCC 12384) or ERY^R/CLI^R (15-003) GAS. Treatments began 4 h post-infection and continued for 72 h. TZD and LNZ (10, 20 and 40 mg/kg) were given intraperitoneally every 12 h. Saline, penicillin (PEN), CLI and ERY were given every 6 h. Survival and infection severity signs and symptoms were followed for 12 days.

RESULTS

Both GAS strains were susceptible to LNZ, TZD and PEN; strain 15-003 was confirmed as constitutively resistant to ERY and CLI. Blood levels following a 40 mg/kg dose of LZD and TZD were 30.9 ± 4.0 μg/mL and 21.9 ± 5.3 μg/mL, respectively. Both TZD and LNZ were highly efficacious for the treatment of severe experimental myonecrosis caused by ERY^S/CLI^S and, importantly, ERY^R/CLI^R GAS.

CONCLUSION

In the current era of emerging macrolide/lincosamide resistance among GAS, these data support the use of TZD and LNZ as first-line antibiotics for the treatment of life-threatening GAS infections in humans.

Infections Caused by Group C and G Streptococcus (Streptococcus dysgalactiae subsp. equisimilis and Others): Epidemiological and Clinical Aspects

Streptococci carrying serogroup C and G antigens, and in particular, Streptococcus dysgalactiae subsp. equisimilis (SDSE), are emerging human pathogens that are increasingly isolated from patients with a myriad of infections that range from mundane to life-threatening. SDSE is microbiologically similar to Streptococcus pyogenes. These streptococci frequently cause infections of the throat and skin and soft tissues. Moreover, they may invade the bloodstream and disseminate widely to many deep tissue sites, including the endocardium. Life-threatening invasive infections due to SDSE, including the streptococcal toxic shock syndrome, occur most frequently in patients with severe underlying medical diseases. Treatment with penicillin is adequate under most circumstances, but treatment failure occurs. SDSE may also be resistant to other antibiotic classes including tetracyclines, macrolides, and clindamycin. Most human infections caused by groups C and G streptococci are transmitted from person to person, but infections due to Streptococcus equi subsp. zooepidemicus (and, rarely, to S. equi subsp. equi) are zoonoses. Transmission of these latter species occurs by animal contact or by contamination of food products and has been associated with the development of poststreptococcal glomerulonephritis. Members of the Streptococcus anginosus group, usually classified with the viridans group of streptococci, are associated with a variety of pyogenic infections.

The effects of iclaprim on exotoxin production in methicillin-resistant and vancomycin-intermediate Staphylococcus aureus

PURPOSE

Extracellular protein toxins contribute to the pathogenesis of Staphylococcus aureus infections. The present study compared the effects of iclaprim and trimethoprim - two folic acid synthesis inhibitors - with nafcillin and vancomycin on production of Panton-Valentine leukocidin (PVL), alpha haemolysin (AH) and toxic-shock syndrome toxin I (TSST-1) in methicillin-resistant and vancomycin-intermediate S. aureus (MRSA and VISA, respectively).

METHODOLOGY

Northern blotting and RT-PCR were used to assess gene transcription; toxin-specific bioassays were used to measure protein toxin production.

RESULTS

As shown previously, sub-inhibitory concentrations (sub-MIC) of nafcillin increased and prolonged MRSA toxin gene transcription and enhanced PVL, TSST-1 and AH production. Sub-inhibitory doses of iclaprim and trimethoprim delayed maximal AH gene (hla) transcription and suppressed AH production; both drugs delayed, but neither reduced, maximal TSST-1 production. Trimethoprim significantly increased lukF-PV expression and PVL production compared to both untreated and iclaprim-treated cultures. Higher concentrations of iclaprim and trimethoprim markedly suppressed MRSA growth, mRNA synthesis and toxin production. In VISA, iclaprim, vancomycin and nafcillin variably increased tst and hla expression, but only nafcillin increased toxin production. Despite its ability to increase hla expression, iclaprim was the most potent inhibitor of AH production.

CONCLUSIONS

We conclude that, due to its ability to suppress toxin production, iclaprim should be effective against severe staphylococcal infections caused by toxin-producing MRSA and VISA strains, especially given its ability to concentrate at sites of infection such as skin and skin structures and the lung.

Physicochemical Factors That Favor Conjugation of an Antibiotic Resistant Plasmid in Non-growing Bacterial Cultures in the Absence and Presence of Antibiotics

Horizontal gene transfer (HGT) of antibiotic resistance genes has received increased scrutiny from the scientific community in recent years owing to the public health threat associated with antibiotic resistant bacteria. Most studies have examined HGT in growing cultures. We examined conjugation in growing and non-growing cultures of E. coli using a conjugative multi antibiotic and metal resistant plasmid to determine physiochemical parameters that favor horizontal gene transfer. The conjugation frequency in growing and non-growing cultures was generally greater under shaken than non-shaken conditions, presumably due to increased frequency of cell collisions. Non-growing cultures in 9.1 mM NaCl had a similar conjugation frequency to that of growing cultures in Luria-Bertaini broth, whereas those in 1 mM or 90.1 mM NaCl were much lower. This salinity effect on conjugation was attributed to differences in cell-cell interactions and conformational changes in cell surface macromolecules. In the presence of antibiotics, the conjugation frequencies of growing cultures did not increase, but in non-growing cultures of 9.1 mM NaCl supplemented with Cefotaxime the conjugation frequency was as much as nine times greater than that of growing cultures. The mechanism responsible for the increased conjugation in non-growing bacteria was attributed to the likely lack of penicillin-binding protein 3 (the target of Cefotaxime), in non-growing cells that enabled Cefotaxime to interact with the plasmid and induce conjugation. Our results suggests that more attention may be owed to HGT in non-growing bacteria as most bacteria in the environment are likely not growing and the proposed mechanism for increased conjugation may not be unique to the bacteria/plasmid system we studied.

Postpartum Invasive Group A Streptococcus Infection: Case Report and Mini-review

The overall incidence of postpartum invasive group A streptococcal (GAS) disease is low in the United States. However, postpartum women are much more likely to develop GAS disease than nonpregnant women. Additionally, postpartum GAS has the potential to develop into a severe disease and a delay in diagnosis can have deadly consequences. This case describes a patient with invasive postpartum endometritis in the setting of diastases of the pubic symphysis. Sepsis secondary to the endometritis develops along with bilateral pneumonia. This case characterizes some of the typical and atypical symptoms a patient with invasive postpartum GAS can present with. Further, it outlines the timely identification of the disease and its appropriate treatment to prevent a potentially disastrous outcome.

Single cell observations show persister cells wake based on ribosome content

Since persister cells survive antibiotic treatments through dormancy and resuscitate to reconstitute infections, it is imperative to determine the rate at which these cells revive. Using two sets of Escherichia coli persister cells, those arising after antibiotic treatment at low levels and those generated at high levels by ceasing transcription via rifampicin pretreatment (shown to be bona fide persisters through eight sets of experiments), we used microscopy of single cells to determine that the resuscitation of dormant persisters is heterogeneous and includes cells that grow immediately. In all, five phenotypes were found during the observation of persister cells when fresh nutrients were added: (i) immediate division, (ii) immediate elongation followed by division, (iii) immediate elongation but no division, (iv) delayed elongation/division and (v) no growth. In addition, once cell division begins, the growth rate is that of exponential cells. Critically, the greater the ribosome content, the faster the persister cells resuscitate.

Bivalent mucosal peptide vaccines administered using the LCP carrier system stimulate protective immune responses against Streptococcus pyogenes infection

Despite the broad knowledge about the pathogenicity of Streptococcus pyogenes there is still a controversy about the correlate of protection in GAS infections. We aimed in further improving the immune responses stimulated against GAS comparing different vaccine formulations including bis-(3',5')-cyclic dimeric adenosine monophosphate (c-di-AMP) and BPPCysMPEG, a derivative of the macrophage-activating lipopeptide (MALP-2), as adjuvants, respectively, to be administered with and without the universal T helper cell epitope P25 along with the optimized B cell epitope J14 of the M protein and B and T cell epitopes of SfbI. Lipopeptide based nano carrier systems (LCP) were used for efficient antigen delivery across the mucosal barrier. The stimulated immune responses were efficient in protecting mice against a respiratory challenge with a lethal dose of a heterologous S. pyogenes strain. Moreover, combination of the LCP based peptide vaccine with c-di-AMP allowed reduction of antigen dose at the same time maintaining vaccine efficacy.

Antimicrobial Effects of β-Lactams on Imipenem-Resistant Ceftazidime-Susceptible Pseudomonas aeruginosa

We studied the resistance mechanism and antimicrobial effects of β-lactams on imipenem-resistant Pseudomonas aeruginosa isolates that were susceptible to ceftazidime as detected by time-kill curve methods. Among 215 P. aeruginosa isolates from hospitalized patients in eight hospitals in the Republic of Korea, 18 isolates (23.4% of 77 imipenem-resistant isolates) were imipenem resistant and ceftazidime susceptible. Multilocus sequence typing revealed diverse genotypes, which indicated independent emergence. These 18 isolates were negative for carbapenemase genes. All 18 imipenem-resistant ceftazidime-susceptible isolates showed decreased mRNA expression of oprD, and overexpression of mexB was observed in 13 isolates. In contrast, overexpression of ampC, mexD, mexF, or mexY was rarely found. Time-kill curve methods were applied to three selected imipenem-resistant ceftazidime-susceptible isolates at a standard inoculum (5 × 10⁵ CFU/ml) or at a high inoculum (5 × 10⁷ CFU/ml) to evaluate the antimicrobial effects of β-lactams. Inoculum effects were detected for all three β-lactam antibiotics, ceftazidime, cefepime, and piperacillin-tazobactam, against all three isolates. The antibiotics had significant killing effects in the standard inoculum, but no effects in the high inoculum were observed. Our results suggest that β-lactam antibiotics should be used with caution in patients with imipenem-resistant ceftazidime-susceptible P. aeruginosa infection, especially in high-inoculum infections such as endocarditis and osteomyelitis.

Toxic Shock Syndrome

Toxic shock syndrome (TSS) represents a heterogeneous group of disorders that results in hypotension, multiorgan system involvement, and a characteristic rash or soft tissue infection caused by staphylococcal or streptococcal exotoxins and enterotoxins. Staphylococcal TSS emerged in the late 1970s as an illness associated with highly absorbent tampons; subsequently it has been described with postoperative infections, burns, and various viral illnesses. Although the morbidity rate associated with staphylococcal TSS may be high, the mortality rate approximates 5%. Streptococcal TSS has emerged in the 1980s and into the 1990s as a disorder that results in rapid progression of soft tissue infection in the form of cellulitis, myositis, or necrotizing fasciitis due to pyogenic streptococcal group A exotoxin. The rapidity of progression of local infection to hypotension and multiorgan failure results in a mortality rate of 30–70%. In both forms of TSS, staphylococcal and streptococcal exotoxins function as superantigens, a unique mechanism of immune activation that results in an exuberant T-cell response and profound cytokine expression. The role of antibiotics is reviewed. The use of clindamycin in streptococcal TSS and the potential therapeutic role of intravenous immunoglobulin in both forms of this disorder are discussed as well.

Necrotizing Fasciitis Caused by Group A Streptococcus: Case Report and Therapy Update

Necrotizing fasciitis is a life-threatening infection that affects the fascia and fat tissue underlying the skin. The diagnosis is often difficult because subcutaneous changes may not be readily apparent. Toxin-producing bacteria are usually the cause, with group A streptococcus (GAS) or Streptococcus pyogenes being responsible for a significant portion of the morbidity and mortality associated with this infection. The mortality rate associated with necrotizing fasciitis varies between 30% and 60%. Toxic shock-like syndrome and multisystem organ failure are the usual causes of death. Early diagnosis and surgery have been associated with decreased morbidity and mortality, and appropriate antimicrobial (eg, penicillin plus clindamycin) and supportive therapy is of utmost importance. Intravenous immunoglobulin and hyperbaric oxygen therapy may be beneficial in treating the infection; however, these 2 therapies require further research. Clinicians need to familiarize themselves with the disease and the different treatment modalities to be able to make the appropriate therapeutic decision. The optimal treatment of necrotizing fasciitis still remains a challenge today. This article presents an illustrative case with a brief overview of necrotizing fasciitis, and the current therapeutic modalities used in the management of the disease.

A novel murine model of Clostridium sordellii myonecrosis: Insights into the pathogenesis of disease

Clostridium sordellii infections have been reported in women following natural childbirth and spontaneous or medically-induced abortion, injection drug users and patients with trauma. Death is rapid and mortality ranges from 70 to 100%. Clinical features include an extreme leukemoid reaction, the absence of fever, and only minimal pain or erythema at the infected site. In the current study, we developed a murine model of C. sordellii soft tissue infection to elucidate the pathogenic mechanisms. Mice received 0.5, 1.0 or 2.0 × 10(6) CFU C. sordellii (ATCC 9714 type strain) in the right thigh muscle. All doses caused fatal infection characterized by intense swelling of the infected limb but no erythema or visible perfusion deficits. Survival rates and time to death were inoculum dose-dependent. Mice developed a granulocytic leukocytosis with left shift, the onset of which directly correlated with disease severity. Histopathology of infected tissue showed widespread edema, moderate muscle damage and minimal neutrophil infiltration. Circulating levels of granulocyte colony-stimulating factor (G-CSF), soluble tumor necrosis factor receptor I (sTNF-RI) and interlukin-6 (IL-6) were significantly increased in infected animals, while TNF-α, and IL-1β levels were only mildly elevated, suggesting these host factors likely mediate the leukocytosis and innate immune dysfunction characteristic of this infection. Thus, this model mimics many of the salient features of this infection in humans and has allowed us to identify novel targets for intervention.

Case Report of Necrotizing Fasciitis Associated with Streptococcus pneumoniae

Necrotizing fasciitis, caused by Streptococcus pneumoniae, is an extremely rare and life-threatening bacterial soft tissue infection. We report a case of early necrotizing fasciitis associated with Streptococcus pneumoniae infection in a 26-year-old man who was immunocompromised with mixed connective tissue disease. The patient presented with acute, painful, erythematous, and edematous skin lesions of his right lower back, which rapidly progressed to the right knee. The patient underwent surgical exploration, and a diagnosis of necrotizing fasciitis was confirmed by pathological evidence of necrosis of the fascia and neutrophil infiltration in tissue biopsies. Cultures of fascial tissue biopsies and blood samples were positive for Streptococcus pneumoniae. To our knowledge, this is the first report of necrotizing fasciitis resulting from Streptococcus pneumoniae diagnosed at early phase; the patient recovered well without surgical debridement.

Classic reaction kinetics can explain complex patterns of antibiotic action

Finding optimal dosing strategies for treating bacterial infections is extremely difficult, and improving therapy requires costly and time-intensive experiments. To date, an incomplete mechanistic understanding of drug effects has limited our ability to make accurate quantitative predictions of drug-mediated bacterial killing and impeded the rational design of antibiotic treatment strategies. Three poorly understood phenomena complicate predictions of antibiotic activity: post-antibiotic growth suppression, density-dependent antibiotic effects, and persister cell formation. We show that chemical binding kinetics alone are sufficient to explain these three phenomena, using single-cell data and time-kill curves of Escherichia coli and Vibrio cholerae exposed to a variety of antibiotics in combination with a theoretical model that links chemical reaction kinetics to bacterial population biology. Our model reproduces existing observations, has a high predictive power across different experimental setups (R(2) = 0.86), and makes several testable predictions, which we verified in new experiments and by analyzing published data from a clinical trial on tuberculosis therapy. Although a variety of biological mechanisms have previously been invoked to explain post-antibiotic growth suppression, density-dependent antibiotic effects, and especially persister cell formation, our findings reveal that a simple model that considers only binding kinetics provides a parsimonious and unifying explanation for these three complex, phenotypically distinct behaviours. Current antibiotic and other chemotherapeutic regimens are often based on trial and error or expert opinion. Our "chemical reaction kinetics"-based approach may inform new strategies, which are based on rational design.

In Vivo Pharmacodynamics of Cefquinome in a Neutropenic Mouse Thigh Model of Streptococcus suis Serotype 2 at Varied Initial Inoculum Sizes

Streptococcus suis serotype 2 is an emerging zoonotic pathogen and causes severe disease in both pigs and human beings. Cefquinome (CEQ), a fourth-generation cephalosporin, exhibits broad-spectrum activity against Gram-positive bacteria such as S. suis. This study evaluated the in vitro and in vivo antimicrobial activities of CEQ against four strains of S. suis serotype 2 in a murine neutropenic thigh infection model. We investigated the effect of varied inoculum sizes (10(6) to 10(8) CFU/thigh) on the pharmacokinetic (PK)/pharmacodynamic (PD) indices and magnitudes of a particular PK/PD index or dose required for efficacy. Dose fractionation studies included total CEQ doses ranging from 0.625 to 640 mg/kg/24 h. Data were analyzed via a maximum effect (Emax) model using nonlinear regression. The PK/PD studies demonstrated that the percentage of time that serum drug levels were above the MIC of free drug (%ƒT>MIC) in a 24-h dosing interval was the primary index driving the efficacy of both inoculum sizes (R(2) = 91% and R(2) = 63%). CEQ doses of 2.5 and 40 mg/kg body weight produced prolonged postantibiotic effects (PAEs) of 2.45 to 8.55 h. Inoculum sizes had a significant influence on CEQ efficacy. Compared to the CEQ exposure and dosages in tests using standard inocula, a 4-fold dose (P = 0.006) and a 2-fold exposure time (P = 0.01) were required for a 1-log kill using large inocula of 10(8) CFU/thigh.

Interactions between Microcystis aeruginosa and coexisting amoxicillin contaminant at different phosphorus levels

Microcystis aeruginosa was cultured with 0.05-5 mg L(-1) of phosphorus and exposed to 200-500 ng L(-1) of amoxicillin for seven days. Amoxicillin presented no significant effect (p>0.05) on the growth of M. aeruginosa at phosphorus levels of 0.05 and 0.2 mg L(-1), but stimulated algal growth as a hormesis effect at phosphorus levels of 1 and 5 mg L(-1). Phosphorus and amoxicillin affected the contents of chlorophyll-a, adenosine triphosphate (ATP) and malondialdehyde, the expression of psbA and rbcL, as well as the activities of adenosinetriphosphatase and glutathione S-transferase in similar manners, but regulated the production and release of microcystins and the activities of superoxide dismutase and peroxidase in different ways. Increased photosynthesis activity was related with the ATP consumption for the stress response to amoxicillin, and the stress response was enhanced as the phosphorus concentration increased. The biodegradation of amoxicillin by M. aeruginosa increased from 11.5% to 28.2% as the phosphorus concentration increased. Coexisting amoxicillin aggravated M. aeruginosa pollution by increasing cell density and concentration of microcystins, while M. aeruginosa alleviated amoxicillin pollution via biodegradation. The interactions between M. aeruginosa and amoxicillin were significantly regulated by phosphorus (p<0.05) and led to a complicated situation of combined pollution.

Infective Endocarditis in Adults: Diagnosis, Antimicrobial Therapy, and Management of Complications: A Scientific Statement for Healthcare Professionals From the American Heart Association

BACKGROUND

Infective endocarditis is a potentially lethal disease that has undergone major changes in both host and pathogen. The epidemiology of infective endocarditis has become more complex with today's myriad healthcare-associated factors that predispose to infection. Moreover, changes in pathogen prevalence, in particular a more common staphylococcal origin, have affected outcomes, which have not improved despite medical and surgical advances.

METHODS AND RESULTS

This statement updates the 2005 iteration, both of which were developed by the American Heart Association under the auspices of the Committee on Rheumatic Fever, Endocarditis, and Kawasaki Disease, Council on Cardiovascular Disease of the Young. It includes an evidence-based system for diagnostic and treatment recommendations used by the American College of Cardiology and the American Heart Association for treatment recommendations.

CONCLUSIONS

Infective endocarditis is a complex disease, and patients with this disease generally require management by a team of physicians and allied health providers with a variety of areas of expertise. The recommendations provided in this document are intended to assist in the management of this uncommon but potentially deadly infection. The clinical variability and complexity in infective endocarditis, however, dictate that these recommendations be used to support and not supplant decisions in individual patient management.

Ante-partum necrotising myometritis due to Streptococcal toxic shock

Group A streptococcus (GAS) causes severe infections in obstetric patients. A rare complication is rapidly progressive necrotising myometritis. Postpartum necrotising myometritis has been previously described; however, antenatal development of such a condition is extremely rare. We present a patient who developed antenatal necrotising myometritis and toxic shock syndrome (TSS) due to GAS during the first trimester of pregnancy, eventually requiring hysterectomy and bilateral oophorectomy. We discuss the rare complication of ante-partum necrotising myometritis, as well as the antibiotic therapy, and treatment of TSS associated with severe Group A Streptococcal infections.

Hormesis effects of amoxicillin on growth and cellular biosynthesis of Microcystis aeruginosa at different nitrogen levels

Coexisting antibiotic contaminants have potential to regulate cyanobacterial bloom, and the regulation is likely affected by nitrogen supply. A typical cyanobaterium Microcystis aeruginosa was cultured with 0.05-50 mg L(-1) of nitrogen and exposed to 100-600 ng L(-1) of amoxicillin for 7 days. Algal growth was not significantly (p > 0.05) affected by amoxicillin at the lowest nitrogen level of 0.05 mg L(-1), stimulated by 600 ng L(-1) of amoxicillin at a moderate nitrogen level of 0.5 mg L(-1) and enhanced by 100-600 ng L(-1) of amoxicillin at higher nitrogen levels of 5-50 mg L(-1). Amoxicillin affected chlorophyll-a, psbA gene, and rbcL gene in a similar manner as algal growth, suggesting a regulation of algal growth via the photosynthesis system. At each nitrogen level, synthesis of protein and polysaccharides as well as production and release of microcystins (MCs) increased in response to environmental stress caused by amoxicillin. Expression of ntcA and mcyB showed a positive correlation with the total content of MCs under exposure to amoxicillin at nitrogen levels of 0.05-50 mg L(-1). Nitrogen and amoxicillin significantly (p < 0.05) interact with each other on the regulation of algal growth, synthesis of chlorophyll-a, production and release of MCs, and expression of ntcA and mcyB. The nitrogen-dependent stimulation effect of coexisting amoxicillin contaminant on M. aeruginosa bloom should be fully considered during the combined pollution control of M. aeruginosa and amoxicillin.

Cellular responses and biodegradation of amoxicillin in Microcystis aeruginosa at different nitrogen levels

The influence of nitrogen on the interactions between amoxicillin and Microcystis aeruginosa was investigated using a 7-day exposure test. Growth of M. aeruginosa was not significantly (p>0.05) affected by amoxicillin at the lowest nitrogen level of 0.05 mg L(-1), stimulated by 500 ng L(-1) of amoxicillin at a moderate nitrogen level of 0.5 mg L(-1) and enhanced by 200-500 ng L(-1) of amoxicillin at the highest nitrogen level of 5 mg L(-1). The generation of reactive oxygen species (ROS) and the synthesis of glutathione S-transferases (GST) and glutathione (GSH) were more sensitive to amoxicillin and were stimulated at all nitrogen levels. At the lowest nitrogen level of 0.05 mg L(-1), superoxide dismutase and peroxidase were not effective at eliminating amoxicillin-induced ROS, resulting in the highest malondialdehyde content in M. aeruginosa. The biodegradation of 18.5-30.5% of amoxicillin by M. aeruginosa was coupled to increasing GST activity and GSH content. Elevated nitrogen concentrations significantly enhanced (p<0.05) the stimulation effect of amoxicillin on the growth of M. aeruginosa, the antioxidant responses to amoxicillin and the biodegradation of amoxicillin in M. aeruginosa. The nitrogen-dependent hormesis effect of the coexisting amoxicillin contaminant on the M. aeruginosa bloom should be fully considered during the control of M. aeruginosa bloom.

Comparison of in vivo and in vitro pharmacodynamics of a humanized regimen of 600 milligrams of Ceftaroline Fosamil every 12 hours against Staphylococcus aureus at initial inocula of 106 and 108 CFU per milliliter

In light of the in vivo/in vitro discordance among beta-lactams against Gram-negative pathogens, we compared the in vivo pharmacodynamics of humanized ceftaroline against 9 Staphylococcus aureus strains (MICs of 0.13 to 1 mg/liter) from published in vitro studies using standard and high inocula in the murine thigh infection model. Consistent with the in vitro findings, mean reductions of ≥1 log10 CFU were observed for ceftaroline against all strains using both standard and high inocula. These results suggest in vivo/in vitro concordance with no observed inoculum effect.

Limitations of beta-lactam therapy for infections caused by susceptible Gram-positive bacteria

Penicillin and related beta-lactam agents have been the most widely used and most important antimicrobials in medical history, and remain the recommended therapy for many infectious diseases 85 years after the discovery of penicillin by Alexander Fleming. Yet the efficacy of these agents has been undermined by two factors - the emergence of clinically significant resistance to the antimicrobial activity of these agents, and clinical situations in which these drugs may be suboptimal (even though the bacterial pathogens are not "resistant" to the drugs). Observations in experimental infection models in animals (group A streptococcal myositis, pneumococcal meningitis and pneumonia, group B streptococcal sepsis) and in some cases clinical studies suggest that monotherapy with beta-lactam antibiotics may be inferior to treatment with other types of antibiotics, alone or in combination with beta-lactams - even in situations where the bacterial pathogens remain fully "susceptible" to beta-lactams in vitro.

Effects of selective and nonselective nonsteroidal anti-inflammatory drugs on antibiotic efficacy of experimental group A streptococcal myonecrosis

BACKGROUND

 Epidemiologic evidence suggests that nonsteroidal anti-inflammatory drugs (NSAIDs) contribute to more severe group A streptococcal (GAS) infections, yet a beneficial role for NSAIDs has been demonstrated in other experimental bacterial infections.

METHODS

 Nonselective (ketorolac tromethamine, ibuprofen, indomethacin), COX-1-selective (SC-560), or COX-2-selective (SC-236) NSAIDs ± antibiotics (penicillin, clindamycin) were given to mice challenged intramuscularly with M-type 3 GAS and disease course was followed for 14 days. RESULTS. All nonselective NSAIDs significantly accelerated mortality and reduced antibiotic efficacy; COX-selective NSAIDs had no significant effects.

CONCLUSIONS

 Use of nonselective NSAIDs, either alone or as adjuncts to antibiotic therapy, for GAS soft tissue infection may contribute to worse outcomes.

Serious group a streptococcal infections

The spectrum of illnesses caused by group A streptococcus (GAS) includes invasive infections, noninvasive infections, and noninfectious complications. Increasingly virulent infections associated with high morbidity and mortality have been observed since the late 1980s and continue to be prevalent in North America and worldwide. Penicillin remains the therapy of choice, with the addition of clindamycin recommended in high risk cases. Early recognition of GAS as the cause of these serious clinical syndromes is critical for timely administration of appropriate therapy. In this review, the pathophysiology, clinical manifestations, and treatment of invasive GAS infections are discussed.

Inoculum effects of ceftobiprole, daptomycin, linezolid, and vancomycin with Staphylococcus aureus and Streptococcus pneumoniae at inocula of 10(5) and 10(7) CFU injected into opposite thighs of neutropenic mice

Reduced bactericidal efficacy at a high inoculum is known as the inoculum effect (IE). We used neutropenic mice to compare the IEs of ceftobiprole (CFB), daptomycin (DAP), linezolid (LZD), and vancomycin (VAN) against 6 to 9 strains of Staphylococcus aureus and 4 strains of Streptococcus pneumoniae at 2 inocula in opposite thighs of the same mice. Neutropenic mice had 10(4.5) to 10(5.7) CFU/thigh (low inoculum [LI]) in one thigh and 10(6.4) to 10(7.2) CFU/thigh (high inoculum [HI]) in the opposite thigh when treated for 24 h with subcutaneous (s.c.) doses every 12 h of DAP at 0.024 to 100 mg/kg of body weight and LZD at 0.313 to 320 mg/kg and s.c. doses every 6 h of CFB at 0.003 to 160 mg/kg and VAN at 0.049 to 800 mg/kg. Dose-response data were analyzed by a maximum effect (E(max)) model using nonlinear regression. Static doses for each drug and at each inoculum were calculated, and the difference between HI and LI (IE index) gave the magnitude of IE for each drug-organism combination. Mean (range) IE indexes of S. aureus were 2.9 (1.7 to 4.6) for CFB, 4.1 (2.6 to 9.3) for DAP, 4.6 (1.7 to 7.1) for LZD, and 10.1 (6.3 to 20.3) for VAN. In S. pneumoniae, the IE indexes were 2.5 (1.3 to 3.3) for CFB, 2.0 (1.6 to 2.8) for DAP, 1.9 (1.7 to 2.2) for LZD, and 1.5 (0.8 to 3.2) for VAN; these values were similar for all drugs. In S. aureus, the IE was much larger with VAN than with CFB, DAM, and LZD (P < 0.05). An in vivo time course of vancomycin activity showed initiation of killing at 4- to 16-fold-higher doses at HI than at LI despite similar initial growth of controls.

High β-lactamase levels change the pharmacodynamics of β-lactam antibiotics in Pseudomonas aeruginosa biofilms

Resistance to β-lactam antibiotics is a frequent problem in Pseudomonas aeruginosa lung infection of cystic fibrosis (CF) patients. This resistance is mainly due to the hyperproduction of chromosomally encoded β-lactamase and biofilm formation. The purpose of this study was to investigate the role of β-lactamase in the pharmacokinetics (PK) and pharmacodynamics (PD) of ceftazidime and imipenem on P. aeruginosa biofilms. P. aeruginosa PAO1 and its corresponding β-lactamase-overproducing mutant, PAΔDDh2Dh3, were used in this study. Biofilms of these two strains in flow chambers, microtiter plates, and on alginate beads were treated with different concentrations of ceftazidime and imipenem. The kinetics of antibiotics on the biofilms was investigated in vitro by time-kill methods. Time-dependent killing of ceftazidime was observed in PAO1 biofilms, but concentration-dependent killing activity of ceftazidime was observed for β-lactamase-overproducing biofilms of P. aeruginosa in all three models. Ceftazidime showed time-dependent killing on planktonic PAO1 and PAΔDDh2Dh3. This difference is probably due to the special distribution and accumulation in the biofilm matrix of β-lactamase, which can hydrolyze the β-lactam antibiotics. The PK/PD indices of the AUC/MBIC and C(max)/MBIC (AUC is the area under concentration-time curve, MBIC is the minimal biofilm-inhibitory concentration, and C(max) is the maximum concentration of drug in serum) are probably the best parameters to describe the effect of ceftazidime in β-lactamase-overproducing P. aeruginosa biofilms. Meanwhile, imipenem showed time-dependent killing on both PAO1 and PAΔDDh2Dh3 biofilms. An inoculum effect of β-lactams was found for both planktonic and biofilm P. aeruginosa cells. The inoculum effect of ceftazidime for the β-lactamase-overproducing mutant PAΔDDh2Dh3 biofilms was more obvious than for PAO1 biofilms, with a requirement of higher antibiotic concentration and a longer period of treatment.

Streptococcus agalactiae toxic shock-like syndrome: two case reports and review of the literature

We present 2 patients with Streptococcus agalactiae toxic shock-like syndrome and review another 11 well-reported cases from the literature. Streptococcal toxic shock-like syndrome is a devastating illness with a high mortality rate, therefore we stress the importance of early supportive management, antimicrobial therapy, and surgical intervention. Toxic shock-like syndrome is likely to be underestimated in patients with invasive Streptococcus agalactiae infection who present with shock. Early diagnosis requires high suspicion of the illness, along with a thorough mucocutaneous examination. Streptococcus agalactiae produces uncharacterized pyrogenic toxins, which explains the ability of the organism to cause toxic shock-like syndrome.

Virulence-suppressing effects of linezolid on methicillin-resistant Staphylococcus aureus: possible contribution to early defervescence

In the present study, immunomodulatory effects of linezolid (LZD) on methicillin-resistance Staphylococcus aureus (MRSA) infections were evaluated. We have retrospectively reviewed treatment effects of LZD on 52 patients with severe MRSA infections. Sixty-four percent of the febrile patients demonstrated significant defervescence within 3 days, despite the presence of positive culture results. We speculated that this finding might be due to early anti-inflammatory effects of LZD, and to investigate this further we initiated in vivo experiments using mice MRSA pneumonia models. Mice were treated with either LZD or vancomycin (VCM) immediately after intranasal administration of MRSA. Bacterial numbers and levels of inflammatory cytokines in the lungs were determined. Although the bacterial burden in the lungs was not apparently different between the two groups, LZD but not VCM treatment significantly reduced induction of inflammatory cytokines in the lungs (P < 0.05). To evaluate whether this anti-inflammatory response was due to suppression of virulence factor expression, filter-sterilized supernatants of MRSA incubated in broth overnight with sub-MICs of LZD were subcutaneously administered to mice. To clarify whether LZD possesses direct host-modulating activity, cytokine responses to the supernatants were examined in mice pretreated with LZD. Interestingly, MRSA solutions prepared in the presence of sub-MICs of LZD revealed significant suppression of interleukin 6 (IL-6) in a dose-dependent manner (P < 0.05), but pretreatment of mice with LZD revealed no changes in cytokines. These findings suggest that sub-MICs of LZD might suppress virulence factors of MRSA, which may be associated with a reduction in endogenous pyrogens. These data may explain at least in part early defervescence observed in LZD-treated individuals.

Nonlinear pharmacokinetics of piperacillin in healthy volunteers--implications for optimal dosage regimens

AIMS

(i) To describe the first-order and mixed-order elimination pathways of piperacillin, (ii) to determine the between occasion variability (BOV) of pharmacokinetic parameters and (iii) to propose optimized dosage regimens.

METHODS

We performed a five-period replicate dose study in four healthy volunteers. Each subject received 4g piperacillin as a single 5min intravenous infusion in each study period. Drug analysis was performed by HPLC. We used NONMEM and S-ADAPT for population pharmacokinetic analysis and Monte Carlo simulation to predict the probability of target attainment (PTA) with a target time of non-protein bound concentration above MIC >50% of the dosing interval.

RESULTS

A model with first-order nonrenal elimination and parallel first-order and mixed-order renal elimination had the best predictive performance. For a 70kg subject we estimated 4.40lh(-1) for nonrenal clearance, 5.70lh(-1) for first-order renal clearance, 170mgh(-1) for V(max) , and 49.7mgl(-1) for K(m) for the mixed-order renal elimination. The BOV was 39% for V(max) , 117% for K(m) , and 8.5% for total clearance. A 30min infusion of 4g every 6h achieved robust (≥90%) PTAs for MICs ≤12mgl(-1) . As an alternative mode of administration, a 5h infusion of 6g every 8h achieved robust PTAs for MICs ≤48mgl(-1) .

CONCLUSIONS

Part of the renal elimination of piperacillin is saturable at clinically used doses. The BOV of total clearance and volume of distribution were low. Prolonged infusions achieved better PTAs compared with shorter infusions at similar daily doses. This benefit was most pronounced for MICs between 12 and 48mgl(-1) .

Mathematical modeling to characterize the inoculum effect

Killing by beta-lactams is well known to be reduced against a dense bacterial population, commonly known as the inoculum effect. However, the underlying mechanism of this phenomenon is not well understood. We proposed a semi-mechanistic mathematical model to account for the reduced in vitro killing observed. Time-kill studies were performed with 4 baseline inocula (ranging from approximately 1 × 10(5) to 1 × 10(8) CFU/ml) of Escherichia coli ATCC 25922 (MIC, 2 mg/liter). Constant but escalating piperacillin concentrations used ranged from 0.25× to 256× MIC. Serial samples were taken over 24 h to quantify viable bacterial burden, and all the killing profiles were mathematically modeled. The inoculum effect was attributed to a reduction of effective drug concentration available for bacterial killing, which was expressed as a function of the baseline inoculum. Biomasses associated with different inocula were examined using a colorimetric method. Despite identical drug-pathogen combinations, the baseline inoculum had a significant impact on bacterial killing. Our proposed mathematical model was unbiased and reasonable in capturing all 28 killing profiles collectively (r(2) = 0.88). Biomass was found to be significantly more after 24 h with a baseline inoculum of 1 × 10(8) CFU/ml, compared to one where the initial inoculum was 1 × 10(5) CFU/ml (P = 0.002). Our results corroborated previous observations that in vitro killing by piperacillin was significantly reduced against a dense bacterial inoculum. This phenomenon can be reasonably captured by our proposed mathematical model, and it may improve prediction of bacterial response to various drug exposures in future investigations.

Attenuation of colistin bactericidal activity by high inoculum of Pseudomonas aeruginosa characterized by a new mechanism-based population pharmacodynamic model

Colistin is increasingly being utilized against Gram-negative pathogens, including Pseudomonas aeruginosa, resistant to all other antibiotics. Since limited data exist regarding killing by colistin at different initial inocula (CFUo), we evaluated killing of Pseudomonas aeruginosa by colistin at several CFUo and developed a mechanism-based mathematical model accommodating a range of CFUo. In vitro time-kill experiments were performed using >or=8 concentrations up to 64 x the MIC of colistin against P. aeruginosa PAO1 and two clinical P. aeruginosa isolates at CFUo of 10(6), 10(8), and 10(9) CFU/ml. Serial samples up to 24 h were simultaneously modeled in the NONMEM VI (results shown) and S-ADAPT software programs. The mathematical model was prospectively "validated" by additional time-kill studies assessing the effect of Ca(2+) and Mg(2+) on killing of PAO1 by colistin. Against PAO1, killing of the susceptible population was 23-fold slower at the 10(9) CFUo and 6-fold slower at the 10(8) CFUo than at the 10(6) CFUo. The model comprised three populations with different second-order killing rate constants (5.72, 0.369, and 0.00210 liters/h/mg). Bacteria were assumed to release signal molecules stimulating a phenotypic change that inhibits killing. The proposed mechanism-based model had a good predictive performance, could describe killing by colistin for all three studied strains and for two literature studies, and performed well in a prospective validation with various concentrations of Ca(2+) and Mg(2+). The extent and rate of killing of P. aeruginosa by colistin were markedly decreased at high CFUo compared to those at low CFUo. This was well described by a mechanism-based mathematical model, which should be further validated using dynamic in vitro models.

Limitations of antibiotic options for invasive infections caused by methicillin-resistant Staphylococcus aureus: is combination therapy the answer?

Invasive infections caused by methicillin-resistant Staphylococcus aureus (MRSA), particularly those involving persistent bacteraemia, necrotizing pneumonia, osteomyelitis and other deep-seated sites of infections, are associated with high mortality and are often difficult to treat. The response to treatment of severe MRSA infection with currently available antibiotics active against MRSA is often unsatisfactory, leading some physicians to resort to combination antibiotic therapy. Now, with the emergence of community-associated MRSA (CA-MRSA) clones that display enhanced virulence potentially related to up-regulated toxin production, the use of adjuvant protein synthesis-inhibiting antibiotics to reduce toxin production also has been advocated by some experts. In this review, we discuss the limitations of antibiotics currently available for the treatment of serious invasive MRSA infections and review the existing literature that examines the potential role of combination therapy in these infections.

Pharmacokinetics and pharmacodynamics of antibacterial agents

This article reviews pharmacodynamics of antibacterial drugs, which can be used to optimize treatment strategies, prevent emergence of resistance and rationalize the determination of antimicrobial susceptibility. Important pharmacodynamic concepts include the requirements for bactericidal therapy for endocarditis and meningitis, for synergistic combinations to treat enterococcal endocarditis or to shorten the course of antimicrobial therapy, for obtaining maximal plasma concentration/minimal inhibitory concentration (MIC) ratios that are greater than 10 or 24 hour-area under the plasma concentration curve (AUC)/MIC ratios that are greater than 100-125 for concentration-dependent agents against gram-negative bacilli and 25-35 against Streptococcus pneumoniae, and for obtaining percent of time that drug levels are greater than the MIC that is at least 40% to 50% of the dosing interval for time-dependent agents.