Problems in in vitro determination of antibiotic tolerance in clinical isolates

Ciprofloxacin resistance and tolerance of Pseudomonas aeruginosa ocular isolates

PURPOSE

Tolerance to antibiotics may occur due to changes in bacterial growth patterns and can be a precursor to development of resistance. However, there is a lack of information on the ability of ocular bacteria isolates to develop tolerance. This paper explores the tolerance to 8 different antibiotics of 61 microbial keratitis isolates of Pseudomonas aeruginosa from Australia and India using the MBC/MIC ratio, with tolerance defined by a ratio ≥ 32, and tolerance to ciprofloxacin by an agar diffusion assay.

METHODS

Antibiotics used were ciprofloxacin, levofloxacin, gentamicin, tobramycin, piperacillin, imipenem, ceftazidime and polymyxin B. Isolates were sourced from microbial keratitis infections in Australia and India. Minimum bactericidal and minimum inhibitory concentration (MBC and MIC) were obtained using broth microdilution and compared to breakpoints from the Clinical Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) to determine bacterial susceptibility. Tolerance was assessed as MBC/MIC ≥ 32. An alternative method for tolerance detection (TD) was assessed with 13P. aeruginosa sensitive isolates by agar disk diffusion assay of ciprofloxacin followed by application of glucose to the agar and observation of re-growth of colonies.

RESULTS

Thirty-three isolates were resistant to imipenem, 20 to ciprofloxacin, 14 to tobramycin and piperacillin, 12 to levofloxacin and ceftazidime, 8 to gentamicin, and 5 to polymyxin B. The percentage of strains resistant to levofloxacin (7 vs 30 %; p = 0.023), gentamicin (0 vs 24 %; p = 0.005) and tobramycin (4 vs 33 %; p = 0.004) was significantly greater in isolates from India.On average, strains from India exhibited notably greater MIC and MBC values compared to strains obtained from Australia. Out of 61 isolates, none displayed an MBC/MIC ratio ≥ 32. However, three sensitive isolates had low tolerance, nine had medium tolerance and one had high tolerance to ciprofloxacin with the TDtest.

CONCLUSIONS

This study used two methods to determine whether P. aeruginosa strains could show tolerance to antibiotics. Using the MBC/MIC criteria no strain was considered tolerant to any of the eight antibiotics used. When 13 strains were tested for tolerance against ciprofloxacin, the most commonly used monotherapy for keratitis, one had high tolerance and nine had medium tolerance. This demonstrates the capacity of P. aeruginosa to develop tolerance which may result in therapeutic failures if inappropriate dosing regimens are used to treat keratitis.

Assessing antibiotic tolerance of Staphylococcus aureus derived directly from patients by the Replica Plating Tolerance Isolation System - REPTIS

Antibiotic tolerant Staphylococcus aureus pose a great challenge to clinicians as well as to microbiological laboratories and are one reason for treatment failure. Antibiotic tolerant strains survive transient antibiotic exposure despite being fully susceptible in vitro. Thus, fast and reliable methods to detect tolerance in the routine microbiology laboratory are urgently required. We therefore evaluated the feasibility of the replica plating tolerance isolation system (REPTIS) to detect antibiotic tolerance in S. aureus isolates derived directly from patients suffering from different types of infections and investigated possible connections to clinical presentations and patient characteristics. One hundred twenty-five S. aureus isolates were included. Replica plating of the original resistance testing plate was used to assess regrowth in the zones of inhibition, indicating antibiotic tolerance. Bacterial regrowth was assessed after 24 and 48 hours of incubation and an overall regrowth score (ORS) was assigned. Regrowth scores were compared to the clinical presentation. Bacterial regrowth was high for most antibiotics targeting protein synthesis and relatively low for antibiotics targeting other cellular functions such as DNA-replication, transcription and cell wall synthesis, with the exception of rifampicin. Isolates with a blaZ penicillinase had lower regrowth in penicillin and ampicillin. Low ORSs were more prevalent among isolates recovered from patients with immunosuppression or methicillin-resistant S. aureus (MRSA) isolates. In conclusion, REPTIS is useful to detect antibiotic tolerance in clinical microbiological routine diagnostics. Further studies should evaluate the impact of rapid detection of antibiotic tolerance as a clinical decision-making tool for tailored antibiotic treatments.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

When antibiotics fail: a clinical and microbiological perspective on antibiotic tolerance and persistence of Staphylococcus aureus

Staphylococcus aureus is a major human pathogen causing a vast array of infections with significant mortality. Its versatile physiology enables it to adapt to various environments. Specific physiological changes are thought to underlie the frequent failure of antimicrobial therapy despite susceptibility in standard microbiological assays. Bacteria capable of surviving high antibiotic concentrations despite having a genetically susceptible background are described as 'antibiotic tolerant'. In this review, we put current knowledge on environmental triggers and molecular mechanisms of increased antibiotic survival of S. aureus into its clinical context. We discuss animal and clinical evidence of its significance and outline strategies to overcome infections with antibiotic-tolerant S. aureus.

Quantifying the matrix of relationships between reduced vancomycin susceptibility phenotypes and outcomes among patients with MRSA bloodstream infections treated with vancomycin

OBJECTIVES

Several phenotypic characteristics of Staphylococcus aureus have been identified as aetiological factors responsible for adverse outcomes among patients receiving vancomycin. However, characterization of the outcomes associated with these reduced vancomycin susceptibility phenotypes (rVSPs) remains largely incomplete and it is unknown if these features contribute to deleterious treatment outcomes alone or in concert. This study described the interrelationship between rVSPs and assessed their individual and combined effects on outcomes among patients who received vancomycin for a methicillin-resistant S. aureus (MRSA) bloodstream infection.

METHODS

An observational study of adult, hospitalized patients with MRSA bloodstream infections who were treated with vancomycin between January 2005 and June 2009 was performed. The rVSPs evaluated included the following: (i) Etest MIC; (ii) broth microdilution MIC; (iii) MBC : MIC ratio; and (iv) heteroresistance to vancomycin by the Etest macromethod. Failure was defined as any of the following: (i) 30 day mortality; (ii) bacteraemia ≥ 7 days on therapy; or (iii) recurrence of MRSA bacteraemia within 60 days of therapy discontinuation.

RESULTS

During the study period, 184 cases met the study criteria and 41.3% met the failure criteria. There was a clear linear exposure-response relationship between the number of these phenotypic markers and outcomes. As the number of phenotypes escalated, the incidence of overall failure increased incrementally by 10%-18%.

CONCLUSIONS

The data suggest that rVSPs contribute to deleterious treatment outcomes in concert.

Vancomycin tolerance in Gram-positive cocci

Vancomycin, a glycopeptide antimicrobial agent, represents the last line of defence against a wide range of multi-resistant Gram-positive pathogens such as enterococci, staphylococci and streptococci. However, vancomycin-resistant enterococci and staphylococci, along with vancomycin-tolerant clinical isolates, are compromising the therapeutic efficacy of vancomycin. It is conceivable that tolerance may emerge during prolonged vancomycin use. It has not been until recently, however, that the molecular basis of this tolerance began to be understood. Superoxide anions might be involved in the bactericidal activity of vancomycin in enterococci, and recent evidence suggests that the stringent response is partly responsible for vancomycin tolerance in Enterococcus faecalis. The mechanism of vancomycin tolerance in Staphylococcus aureus and Streptococcus pneumoniae is sometimes associated with a reduction of autolysin activity. Vancomycin tolerance in S. aureus and S. pneumoniae also appears to be somehow related with the two-component regulatory systems linked to cell envelope stress, although the precise molecular regulatory pathways remain poorly defined.

[Tolerance and heteroresistance in Gram-positive microorganisms]

During the last few years, insufficient efficacy of currently recommended antimicrobial agents has been observed, mainly in the case of glycopeptides, during the treatment of infections due to methicillin-resistant Staphylococcus aureus, even if these isolates show MIC values within the susceptible range. The phenomena associated with this observation are tolerance (a genetic event in which a bactericidal antibiotic fails to kill a bacterial population), persistence (a non-inherited and transient phenotypic phenomenon in which a bacterial subpopulation -0.1%-10%- survive lethal antimicrobial concentrations irrespective of the mechanisms of action) and heteroresistance (an epigenetic event in which less susceptible isogenic subpopulations are recovered when the entire population is challenged with concentrations exceeding MIC values). New antimicrobials, including daptomycin, are less affected by these phenomena and should be considered as the treatment of choice when these events are demonstrated or suspected.

Inhibitory and bactericidal activities of daptomycin, vancomycin, and teicoplanin against methicillin-resistant Staphylococcus aureus isolates collected from 1985 to 2007

The inhibitory and bactericidal activities of daptomycin, vancomycin, and teicoplanin against a collection of 479 methicillin-resistant Staphylococcus aureus isolates were assessed. The isolates were collected from U.S. and European hospitals from 1985 to 2007 and were primarily from blood and abscess cultures. The MICs and minimum bactericidal concentrations (MBCs) of the three agents were determined, and the MBC/MIC ratios were calculated to determine the presence or absence of tolerance. Tolerance was defined as an MBC/MIC ratio of > or = 32 or an MBC/MIC ratio of > or = 16 when the MBC was greater than or equal to the breakpoint for resistance. Tolerance to vancomycin and teicoplanin was observed in 6.1% and 18.8% of the strains, respectively. Tolerance to daptomycin was not observed.

Endocarditis caused by Staphylococcus aureus: A reappraisal of the epidemiologic, clinical, and pathologic manifestations with analysis of factors determining outcome

Staphylococcus aureus is the leading cause of infectious endocarditis and its mortality has remained high despite better diagnostic and therapeutic procedures over time. We conducted a retrospective review of 133 cases of definite S. aureus endocarditis seen at a single tertiary care hospital over 22 years to assess changes in the epidemiology and incidence of the infection, manifestations, outcome, risk factors for mortality, and impact of cardiac surgery on prognosis.Patients were classified into 2 groups: 1) right-sided endocarditis (64 patients) and 2) left-sided endocarditis (69 patients). While the number of cases of left-sided endocarditis remained steady at 1-3 cases per 10,000 admissions, the incidence of right-sided endocarditis, after a peak in the early 1990s, declined to almost disappear in 2001. Among the cases of right-sided endocarditis, we found 2 subsets of patients with different clinical features and prognosis: the first subset comprised 53 intravenous drug abusers, and the second subset comprised 11 patients with catheter-associated S. aureus bacteremia and endocarditis. Fifty-one patients were human immunodeficiency virus (HIV)-positive drug abusers, most of whom (80.3%) had right-sided endocarditis. We did not find differences in mortality between HIV-positive and HIV-negative individuals; mortality seemed to depend more on the site of the heart involved than on HIV status.Among the cases of left-sided endocarditis, the mitral valve was more commonly involved than the aortic valve (61% vs. 30%). Overall, 74% of patients with left-sided endocarditis developed 1 or more cardiac or extracardiac complication. In comparison, only 23.4% of patients with right-sided endocarditis developed complications.Prosthetic valve endocarditis (PVE) was hospital-acquired more frequently than native valve endocarditis (NVE). Patients with PVE had a shorter duration of symptoms until diagnosis and presented with or developed cardiac murmurs less frequently than patients with NVE. Cardiac failure (49%), renal failure (43%) and central nervous system (CNS) events (35%) were frequently observed in patients with both PVE and NVE. Valve replacement was more frequently needed and more rapidly performed in patients with PVE than in their counterparts with NVE.The overall mortality of patients with right-sided endocarditis was 17%. While the mortality of right-sided endocarditis in injection drug users was 3.7%, the mortality of patients with right-sided endocarditis associated with infected intravenous catheters was 82% (odds ratio [OR], 0.01; 95% confidence interval [CI], 0.001-0.07). For left-sided endocarditis mortality was 38% and was not significantly different in patients with NVE or PVE (OR, 0.65; 95% CI, 0.23-1.87). CNS complications were associated with mortality in both NVE (OR, 6.55; 95% CI, 1.78-24.04) and PVE (OR, 32; 95% CI, 2.63-465.40). Development of 2 or 3 complications was associated with an increased risk of mortality (OR, 5.59; 95% CI, 1.08-28.80 and OR, 9.25; 95% CI, 1.36-62.72 for 2 vs. 1 complication and for 3 vs. 2 complications, respectively).Surgical treatment did not significantly influence mortality in cases of NVE, (OR, 3.19; 95% CI, 0.76-13.38) but significantly improved the prognosis of patients with PVE (OR, 69; 95% CI, 2.89-1647.18).S. aureus endocarditis is an aggressive, often fatal, infection. The results of the current study suggest that valve replacement will improve the outcome of infection, particularly in patients with PVE.

Antimicrobial activity of garlic against oral streptococci

The antimicrobial activity of two garlic clones' (1: purple and 2: white) crude extracts against oral microbiota was evaluated in vitro (study 1) and in vivo (study 2). Study 1 consisted of the evaluation of minimum inhibitory (MIC) and bactericidal (MBC) concentrations against nine streptococci strains. In study 2, a 2.5% garlic (clone 2) solution was used as a mouthwash in a 5-week study by 30 subjects. Blood agar and Mitis Salivarius Bacitracin agar were inoculated with subjects' saliva to quantify oral microorganisms and mutans streptococci. Study 1 showed MIC ranging from 0.5 to 32.0 mg ml(-1) for clone 2 and from 8 to 64.0 mg ml(-1) for clone 1. MBC ranged from 1.0 to 128.0 mg ml(-1) and from 8.0 to 128.0 mg ml(-1) regarding clones 2 and 1 respectively. Study 2 showed that 2.5% garlic mouthwash solution had good antimicrobial activity against mutans streptococci and oral microorganisms. Maintenance of reduced salivary levels of streptococci was observed after 2 weeks at the end of mouthwash use. Unpleasant taste (100%), halitosis (90%) and nausea (30%) were reported by subjects after the end of the study. It was concluded that the garlic clones have antimicrobial properties in vitro against streptococci and anticariogenic properties against oral microorganism in spite of its adverse effects.

Tolerance to the glycopeptides vancomycin and teicoplanin in coagulase-negative staphylococci

Tolerance to vancomycin and teicoplanin in 90 clinical isolates of coagulase-negative staphylococci (CoNS) was investigated by time-kill curve methodology. Only six strains, belonging to the Staphylococcus lugdunensis species, exhibited tolerance. The seven other S. lugdunensis strains tested displayed weak susceptibility to the bactericidal activity of glycopeptides compared to the other CoNS. These phenomena are of concern, since S. lugdunensis is recognized as one of the most pathogenic CoNS.

Microbiological features of vancomycin in the 21st century: minimum inhibitory concentration creep, bactericidal/static activity, and applied breakpoints to predict clinical outcomes or detect resistant strains

The results of vancomycin susceptibility tests document that the drug continues to have activity against a wide variety of gram-positive pathogens. The subsequent emergence of vancomycin-resistant enterococci, the persistent failure of vancomycin therapy against strains tested as susceptible, and the more recent discoveries of vancomycin-intermediate or -resistant Staphylococcus aureus strains have compromised the use of vancomycin. Although analyses of surveillance studies fail to demonstrate "minimum inhibitory concentration creep" among populations of wild-type enterococci, streptococci, or staphylococci, enterococci with acquired resistance to vancomycin continue to evolve. The dominantly used automated commercial tests poorly recognize vancomycin-intermediate S. aureus, heteroresistant vancomycin-intermediate S. aureus, and vancomycin-resistant S. aureus isolates, which necessitates the use of expensive supplemental screening tests. Monitoring for appropriate serum levels of vancomycin and determinations of the bactericidal activity of vancomycin appear to best predict clinical outcome, thus creating additional diagnostic burdens for clinical laboratories. Improvements in current test methods with breakpoint criteria and expanded use of the vancomycin bactericidal assays to detect "tolerant" strains will be required to increase the value of vancomycin treatment or to refocus therapy toward the use of newer, alternative agents.

Determination of fungicidal activities against yeasts and molds: lessons learned from bactericidal testing and the need for standardization

In certain unique clinical settings, the ability of the antimicrobial agent administered to kill the pathogen outright may be quite important. These situations invariably involve infection of a site not easily accessed by host defenses and/or of a structure with essential anatomic or physiologic function such as the heart (endocarditis), central nervous system (meningitis), or bone (osteomyelitis). Likewise, infections in immunosuppressed hosts, especially those who are neutropenic, are often thought to require microbicidal therapy. Proof of the cidal nature of an antimicrobial agent in vitro is tedious, complex, and fraught with error. Although several methods for assessing in vitro bactericidal activity have been standardized (NCCLS M26-A and M21-A), the clinical relevance of these determinations is questionable and the tests are performed infrequently in most laboratories. Most of the clinical data supporting the need for microbicidal therapy and testing have focused on bacterial infections. However, given the fact that most serious fungal infections occur in profoundly immunosuppressed individuals, it is generally assumed that a cidal regimen would be preferable in that setting as well. In view of this clinical concern and the perceived need to assess the fungicidal activity of a variety of agents, we considered that it would be useful to review what is known about the issues and problems in assessing bactericidal activity and the clinical utility of such measurements. Following this review, we discuss the issue of how one defines fungicidal activity in vitro and in vivo and how feasible it might be to determine the fungicidal activity of organism-drug combinations for purposes of both drug development and clinical care. Proposed methods for fungal time-kill determinations and minimal fungicidal concentration determinations are also discussed.

Clinical relevance of bacteriostatic versus bactericidal mechanisms of action in the treatment of Gram-positive bacterial infections

The distinction between bactericidal and bacteriostatic agents appears to be clear according to the in vitro definition, but this only applies under strict laboratory conditions and is inconsistent for a particular agent against all bacteria. The distinction is more arbitrary when agents are categorized in clinical situations. The supposed superiority of bactericidal agents over bacteriostatic agents is of little relevance when treating the vast majority of infections with gram-positive bacteria, particularly in patients with uncomplicated infections and noncompromised immune systems. Bacteriostatic agents (e.g., chloramphenicol, clindamycin, and linezolid) have been effectively used for treatment of endocarditis, meningitis, and osteomyelitis--indications that are often considered to require bactericidal activity. Although bacteriostatic/bactericidal data may provide valuable information on the potential action of antibacterial agents in vitro, it is necessary to combine this information with pharmacokinetic and pharmacodynamic data to provide more meaningful prediction of efficacy in vivo. The ultimate guide to treatment of any infection must be clinical outcome.

Antibiotic persistence: the role of spontaneous DNA repair response

Persisters are a small proportion of a bacterial population that exists in a physiological state permitting survival despite the lethal activity of antibiotics. To explain this phenomenon, it has been suggested that persisters are bacteria repairing spontaneous errors of DNA synthesis. To verify this assumption, Escherichia coli AB1157 and its lexA3 derivative were exposed to a dose 6x MIC of various antibiotics representative of different molecular mechanisms of action (ampicillin, ceftriaxone, meropenem, amikacin, ciprofloxacin). Bacterial cell counts, after 24 hr of exposure to the antimicrobials, revealed a reduction of about 90% of viable organisms in the lexA3 strains in comparison to the lexA+. In several cases, the number of colony-forming units decreased below the limit of assay. This behavior was noted with all antibiotics used, alone or in combination (amikacin plus ceftriaxone and amikacin plus ciprofloxacin). The same experiments were repeated using E. coli AB1157 cultured in the presence of mitomycin C (0.25x MIC), and the number of survivors exceeded by about 90% the values found in the nonexposed control. In contrast, in the sulA background, mitomycin C reacted synergically with all the antibiotics tested causing a strong reduction of the survivors in comparison with the control. The addition of chloramphenicol (0.125x MIC), on the contrary, caused a reduction of the number of survivors of about 90%. These findings indicate that, when DNA repair is active (a mechanism known to block cell division), the number of survivors is greater than that observed with lexA3. Thus, in addition to other possible explanations, persisters might be a fraction of bacteria that during antibiotic treatment are not growing because they are repairing spontaneous errors of DNA synthesis.

Amoxycillin tolerance in Helicobacter pylori

Resistance to amoxycillin in Helicobacter pylori has only recently been reported. To demonstrate the existence of resistance, and to test for the presence of tolerance, 17 amoxycillin-resistant strains of H. pylori, first isolated in Sardinia (Italy) and the USA, were studied. Four amoxycillin-sensitive strains were used as controls. Primary isolates of all test strains exhibited amoxycillin resistance; beta-lactamase activity was not detected. Amoxycillin resistance was lost after storage of strains at -80 degrees C but could be rescued by plating these strains on to amoxycillin gradient plates. MICs and MBCs from rescued isolates ranged from 0.5 to 32 mg/L and from 32 to > 1024 mg/L, respectively. MBC/MIC ratios > or = 32 are characteristic of antibiotic tolerance. The ratios of MBC/MIC of amoxycillin ranged from 32 to > 1024 for the test strains, indicating that these strains were tolerant to the antibiotic. Amoxycillin resistance does occur in H. pylori. Amoxycillin susceptibility testing of H. pylori isolates in patients who fail therapy should include determination of the MBC to detect tolerance.

Isolation, in vitro antimicrobial susceptibility and penicillin tolerance of Arcanobacterium haemolyticum in a Turkish university hospital

Arcanobacterium haemolyticum (Ah) was isolated from 5 (0.3%) out of 1531 throat cultures of patients with presumed pharyngotonsillitis. The age of the patients who had a positive culture for Ah varied between 6 and 22. The isolation rate of beta-haemolytic streptococci (BHS) was 7.4%, 72.6% of which belonged to Group A, followed by groups G, C and B. None of the throat samples yielded simultaneous growth of Ah and BHS. Antimicrobial susceptibility of Ah isolates to phenoxymethylpenicillin, cephalexin, cefotaxime, vancomycin, erythromycin, azithromycin, doxycycline, ciprofloxacin, and trimethoprim-sulfamethoxazole was tested by the agar dilution method. The isolates were found to be susceptible to all antimicrobials tested except trimethoprim-sulfamethoxazole. Penicillin tolerance could be detected in none of the Ah strains, including the reference strain Ah ATCC 9345. We conclude that Ah should be kept in mind as a potential pathogen causing pharyngitis in adolescents and young adults.

Microbiological tolerance in orthopaedic infections: delayed response of septic arthritis and osteomyelitis of the hip due to infection with tolerant Staphylococcus aureus

A 14-year-old boy presented to another hospital with a clinical picture of septic arthritis. After aspiration of purulent material from the joint, empiric antibiotic treatment was initiated and an arthrotomy was performed. Antibiotic treatment was then modified to nafcillin according to microbiological sensitivity results of the isolated Staphylococcus aureus as determined by minimal inhibitory concentration testing. One week later purulent drainage recurred and open drainage had to be repeated; an abscess anterior to the joint was noted. Once again the infection failed to resolve, and the patient was transferred to our institution where a third arthrotomy had to be performed. The organism isolated at the first aspiration was reexamined and found to have a minimal bactericidal concentration to minimal inhibitory concentration ratio of 32, implying a tolerant organism. The antibiotic treatment was modified to an antibiotic not subject to the tolerance phenomenon, and the infection resolved without additional surgical intervention.

Penicillin tolerance and treatment failure in group A streptococcal pharyngotonsillitis

Failure of treatment of group A streptococcal pharyngitis and tonsillitis is well documented. One of the possible explanations for treatment failure is penicillin tolerance in group A streptococci. Reports on the prevalence of penicillin tolerance among group A streptococci (0-100%) and the presumed relationship with therapeutic failure vary considerably. Therefore, it appears worthwhile to review pharyngotonsillitis studies, devoting special attention to the variables of MIC-MBC laboratory determinations such as inoculum preparation, composition and volume of test medium, and the criteria used to define penicillin tolerance. Alternative methods (gradient-replica plate method, beta-lactamase disk test, time-kill assay, and cell-lysis assay) are discussed. It is concluded that technical factors and the definitions used influenced the reported rates of penicillin tolerance. The epidemiological data suggest that tolerance is not limited to a single streptococcal serotype. Furthermore, there is not sufficient data to support a correlation between in vitro penicillin tolerance of group A streptococci and treatment failure, either in clinical cases or in animal studies. On the other hand, evidence to exclude penicillin tolerance as a cause of treatment failure is also not available. Therefore, at present penicillin tolerance cannot be ruled out as a cause of penicillin treatment failures.

Roles of divalent cations and pH in mechanism of action of nitroxoline against Escherichia coli strains

The antibacterial activity of nitroxoline (NIT), an antibiotic used in the treatment of acute or recurrent urinary tract infections caused by Escherichia coli, is decreased in the presence of Mg2+ and Mn2+ but not Ca2+. In order to elucidate the interaction between this drug and the divalent cations, spectrophotometric studies based on the natural absorption of the nitroxoline moiety were conducted. In the presence of the divalent metal ions, a shift in the NIT A448 suggested the formation of drug-ion complexes, for which the stability followed the order Mn2+ > Mg2+ > Ca2+. A clear correlation was found between the chelating property and antibacterial activity of NIT; both were pH dependent. A convenient colorimetric method for the determination of NIT uptake by bacterial cells was also developed. Uptake was energy independent and showed biphasic kinetics: a rapid association with cells and then a slower increase in cell-associated NIT which reached a plateau. NIT uptake was reduced in the presence of magnesium. The implications of metal ion complexation and pH on the clinical efficacy of NIT are discussed.

Discrepancies between MIC and MLC values of amphotericin B against isolates of Aspergillus species

There is little information addressing the phenomena of discrepancy between minimal inhibitory concentrations (MIC) and minimal lethal concentrations (MLC) values of amphotericin B (AMB) to clinical isolates of fungi. This study assessed in vitro activity of AMB against 70 clinical isolates of aspergilli: 30 strains of Aspergillus fumigatus, 20 strains of Aspergillus flavus and 20 strains of Aspergillus niger. Susceptibility tests were accomplished using a macro broth dilution procedure, with special emphasis on ascertainment of MLCs. AMB exhibited low MIC values against all clinical isolates. While we did not identify any AMB resistant isolates among 70 Aspergillus spp. studied as judged by MIC levels, analysis of the data demonstrated a clear discrepancy between the MIC and MLC levels of AMB obtained against clinical isolates of Aspergillus spp. The MLC values of AMB were significantly higher than the MIC values with MIC 50 and MIC 90 of 0.29 and 0.5 microgram/ml, respectively, at the second reading time, and MLC 50 and MLC 90 of 2.31 and 9.24 micrograms/ml, respectively (p < 0.001). Additionally, minimal lethal concentrations in 36/70 (51%) of aspergillal isolates studied produced drug concentrations above those which can usually be sustained in patient plasma or tissue.

In vitro development and stability of tolerance to cloxacillin and vancomycin in Staphylococcus aureus

The stability of tolerance of Staphylococcus aureus during subculturing at 37 degrees C and development of this property after repeated exposure to cloxacillin or vancomycin were investigated in vitro. Four of five tolerant strains lost this property during repeated subculturing at 37 degrees C for 50 days. Conversely, tolerance emerged in two of four nontolerant strains after repeated cycles of exposure to 25 micrograms of cloxacillin/ml or 10 micrograms of vancomycin/ml alternating with growth in antibiotic-free medium. Previous in vivo exposure to cloxacillin did not enhance the development of tolerance in vitro. MICs of both cloxacillin and vancomycin did not change significantly during this procedure. Whether the conversion of nontolerant strains to the tolerant state can also occur during antibiotic exposure in treatment of patients remains to be determined.

The role of new antibiotics for the treatment of infections in the emergency department

Treatment of infectious diseases comprises a large part of emergency medicine practice. The management of infectious diseases in the emergency department is often different than in other settings because of the types of infectious presentations and the frequent necessity of empiric treatment decisions. This paper examines the new antimicrobials and their role for the treatment of infections in the ED.

Antibiotic resistance and penicillin tolerance in clinical isolates of group B streptococci

The aim of this study was to determine the susceptibility patterns of 100 group B streptococcal strains isolated in our hospital and to ascertain tolerance to penicillin by determining quantitative killing curves. We found two strains with intermediate susceptibility to penicillin and eight strains to ampicillin. Seventeen isolates were tolerant to penicillin, with bacterial counts decreasing 2 to 3 log during the first 8 h but still above 10(2) CFU/ml after 24 h. The kinetic study shows that penicillin tolerance is not rare among group B streptococci isolated in our hospital.

Effect of selected antibiotics on Staphylococcus aureus present in milk from infected mammary glands

Milk from a mammary gland infected with Staphylococcus aureus Newbould 305 was collected and dispensed into sterile tubes. Selected antibiotics were added at 2, 4, or 32 times the MIC, and the percentage survival of S. aureus at various times through 24 h after addition of each antibiotic was determined. Percentage survival of the same S. aureus strain grown in vitro and added to 1) Mueller-Hinton broth and 2) ultrahigh temperature pasteurized milk with the same concentration of antibiotics was also determined. Percentage survival observed after 24 h in milk from the infected quarter at 2, 4, and 32 times MIC, respectively, were: penicillin: 12, 9.4, and 13%; cephapirin 43, 50, and 30%; erythromycin: 120, 95, and 82%; pirlimycin: > 1000, 148, and 38%; tilmicosin: > 1000, > 1000, and 9%; ciprofloxacin: 458, 6, and 3%; norfloxacin: 40, 10, and < 1%; rifampicin: < 1, < 1, and < 1%; and novobiocin: 20, 41, and 5%. By comparison, percentage survival observed after 24 h for in vitro grown S. aureus tested in Mueller-Hinton broth at 2, 4, and 32 times MIC, respectively, were < 1% for penicillin, cephapirin, pirlimycin, ciprofloxacin, norfloxacin, and rifampicin, and < 10% for erythromycin, novobiocin, and tilmicosin. UHT milk had minimal effect on percentage survival compared to Mueller-Hinton broth for S. aureus 305 when tested against penicillin or cephapirin. Increased survival was noted in UHT milk for erythromycin, pirlimycin, and tilmicosin at all antibiotic concentrations and for ciprofloxacin, norfloxacin, rifampicin, and novobiocin at 2 times the MIC.(ABSTRACT TRUNCATED AT 250 WORDS)

Tests for bactericidal effects of antimicrobial agents: technical performance and clinical relevance

Bactericidal testing has been used for several decades as a guide for antimicrobial therapy of serious infections. Such testing is most frequently performed when bactericidal antimicrobial agent therapy is considered necessary (such as when treating infectious endocarditis or infection in an immunocompromised host). It has also been used to ensure that the infecting organism is killed by (not tolerant to) usually bactericidal compounds. However, few data are available to support the role of such tests in direct patient care. Several important variables affect the reproducibility of the test results; however, proposed reference methods are now available for performing the MBC test. With minor modifications, these can provide a standardized approach for laboratories that need to perform them. Currently, little evidence is available to support the routine use of such testing for the care of individual patients. However, testing of new (investigational) antimicrobial agents can be beneficial in determining their potential to provide bactericidal antimicrobial activity during clinical use. New methods to assess bactericidal activity are being developed, but as yet none have been rigorously tested in patient care settings; further, for most of these methods, little information is available as to which technical parameters affect their results. In clinical laboratories, all bactericidal tests must be performed with rigorously standardized techniques and adequate controls, bearing in mind the limitations of the currently available test procedures.

Comparative kill and growth rates determined with cefdinir and cefaclor and with Streptococcus pneumoniae and beta-lactamase-producing Haemophilus influenzae

The relationship between the growth rate and the kill rate was used to evaluate and to compare the in vitro bactericidal activities of cefdinir, a new oral cephalosporin, and cefaclor against Streptococcus pneumoniae and beta-lactamase-producing strains of Haemophilus influenzae. These frequently encountered pathogens of community-acquired respiratory tract infections are usually susceptible to both drugs. The MIC ranges for cefdinir and cefaclor were, respectively, 0.03 to 0.06 and 0.25 to 0.5 micrograms/ml for S. pneumoniae and 0.25 and 4 to 8 micrograms/ml for H. influenzae. The colony counts (CFU per milliliter) measured after 6 h of exposure to a range of antibiotic concentrations in broth were plotted against the colony count of the control culture over the same period of time. Higher kill rates versus bacterial growth rates were noted for S. pneumoniae for both drugs (positive balance). Conversely, lower kill rates versus growth rates were noted for H. influenzae for both drugs (negative balance). In conclusion, the bactericidal activities of both drugs against S. pneumoniae and H. influenzae were similar when expressed by the relationship between the growth rate and the kill rate at 6 h, but cefdinir was more active at lower concentrations.

Susceptibility testing of Listeria monocytogenes. A reassessment of bactericidal activity as a predictor for clinical outcome

In vitro susceptibility testing of Listeria monocytogenes most often reveals both ampicillin and penicillin as inhibitory as opposed to bactericidal with activity comparable to chloramphenicol and tetracycline. Yet, the former two penicillins are more effective for Listeria meningitis than are the latter agents. Accordingly, we reassessed the bactericidal activity of agents used in listeriosis in order to determine in vitro methodology that would be more predictive of clinical outcome. We found that bactericidal activity for greater than 48 hr by either minimum inhibitory-minimum bactericidal concentration (MIC-MBC) testing or time-kill kinetic studies was the best predictor of clinical efficacy. This correlation may be due to Listeria being a slow-growing microorganism. In addition to ampicillin and penicillin, we found trimethoprim-sulfamethoxazole, vancomycin, and imipenem to exhibit bactericidal activity for 48 hr. For the first two agents, this is in agreement with the results of clinical experience.

Incidence of penicillin tolerance among blood culture isolates of Streptococcus sanguis, 1987-88

Laboratories that reported isolations of Streptococcus sanguis from blood cultures to the Communicable Disease Surveillance Centre (CDSC) Colindale were requested to submit strains to Bath Public Health Laboratory to allow the prevalence of penicillin tolerance within different biotypes of this species to be studied. One hundred and fifty one Streptococcus spp were received from 78 United Kingdom laboratories in one year. Strains were identified using the API 20 Strep, and minimum inhibitory concentrations (MICs) of penicillin were determined using the spiral gradient plate method. Penicillin tolerance was detected by spraying beta-lactamase over inoculated gradient plates, reincubating for 48 hours and counting the number of surviving organisms represented by colonies. There were 57 different API identification profiles encountered in the survey. Most S sanguis I/1 strains were penicillin tolerant, most S sanguis II strains were non-tolerant. The overall geometric mean MIC of penicillin was considerably lower for S sanguis I/1 than for all other biotypes. The distribution of biotypes and the geometric mean MIC of penicillin for each biotype were not significantly different for infective endocarditis strains than for all strains tested, suggesting little or no association between penicillin tolerance and the seeding of endocardium. When the reactions obtained using API 20 Strep were compared with a recent taxonomic study of viridans streptococci, 22 of 38 S sanguis I/1 strains could be reclassified as S gordonii; all these strains were penicillin tolerant. Such reclassification would allow likely penicillin tolerant strains to be predicted.

Evaluation of two methods for overcoming the antibiotic carry-over effect

The antibiotic carry-over effect occurs when antibiotic transferred onto the agar plate with the subcultured aliquot is sufficient to inhibit the growth of viable bacteria, and results in a falsely low MBC. This phenomenon was eliminated by widely streaking the transferred aliquot over at least one half of a 100 mm agar plate or by centrifugation and resuspension of the organisms in non-antibiotic-containing media prior to plating. Both methods require more effort than the current method of MBC determination, but can be performed in clinical laboratories and are thus highly recommended in the testing of organisms from endocarditis and meningitis patients.

In vitro activity of vancomycin against enterococci

The in vitro activity of vancomycin against 40 clinical isolates of enterococci was determined by a macro-tube dilution method and by quantitative killing curve procedures employing the standard medium of our department, i.e. a filtered ox broth. An attempt to remove the influence of technical factors on the MBC determination was made by using an inoculum in the early logarithmic growth phase and ensuring the exposure of all the organisms to the antibiotic. Vancomycin showed a good inhibitory activity for the enterococci tested (MIC90 of 1.6 micrograms/ml, 3.1 micrograms/ml and 1.6 micrograms/ml for S. faecalis, S. faecium and S. durans, respectively), but no bactericidal effect could be demonstrated as measured by the MBCs (greater than 100 micrograms/ml) and killing curve procedures.

Degree and stability of tolerance to penicillin in Streptococcus pyogenes

The degree of antibiotic tolerance may be assessed by determining the tolerance percentage of a bacterial strain, defined as the surviving fraction of an inoculum that has been exposed for 24 hours to a high concentration of a beta-lactam antibiotic. In 61 clinical isolates of Streptococcus pyogenes, tolerance percentages ranged from 0 to 0.43. From the slopes of the killing curves it can be deduced that killing starts to be delayed at a tolerance percentage of 0.1. Although a tolerance percentage exceeding 0.1 was observed in 41.4% of the strains, the incidence of clinically relevant forms of tolerance is expected to occur in a smaller fraction of the strains. Tolerance percentages of two strains stored at 20 degrees C, 4 degrees C or -70 degrees C (tolerance percentages 0.43 and 0.36) decreased to 0.03 or less in six weeks. Tolerance percentages could be completely restored in these strains, but not in sensitive strains, by successive selection for this property on penicillin gradients of increasing concentration. In four strains isolated from a family outbreak, identical levels of tolerance percentage could be selected for with the same technique.

Tolerance to penicillin in streptococci of viridans group

Tolerance to penicillin was investigated in 40 isolates of optochin-resistant, alpha-haemolytic streptococci. Thirteen strains exhibited tolerance to penicillin (MBC:MIC ratio greater than or equal to 32) when stationary phase inocula were used, but only seven strains retained the tolerance phenotype in experiments with logarithmic phase inocula. There was a striking association between tolerance and Eagle's optimum dosage effect, particularly among strains that displayed tolerance in both the stationary and the logarithmic growth phases. Sequential viable counts on representative strains showed that reliance on the arbitrary criterion of bactericidal activity of 99.9% reduction of the original inoculum after 24 hours' exposure may occasionally lead to difficulties in the recognition of penicillin tolerance. In general, however, the 99.9% killing criterion provided a useful discriminator between strains that were rapidly killed by penicillin and those (tolerant strains) in which the bactericidal activity was much reduced.

Bactericidal activity of oxacillin against beta-lactamase-hyperproducing Staphylococcus aureus

The bactericidal activity of oxacillin against beta-lactamase-hyperproducing strains of Staphylococcus aureus for which the MIC by MicroScan was 1 or 2 micrograms/ml after incubation for 24 h was evaluated by MBC studies and kill kinetics methods. MBC and kill kinetics tests were both performed using Mueller-Hinton broth (MHB), with and without 2% NaCl supplementation, and incubation at 30 and 35 degrees C. When MBC testing was performed with salt-supplemented MHB, the oxacillin MBC/MIC ratio was greater than 8 for 17 and 16 of 17 S. aureus isolates at 30 and 35 degrees C, respectively. With unsupplemented MHB, the MBC/MIC ratio was greater than 8 for nine and six strains at 30 and 35 degrees C, respectively. Five representative strains were selected for kill kinetics studies under the four different test conditions. Oxacillin appeared more bactericidal by the kill kinetics method than by MBC testing. Moreover, salt supplementation did not affect the results of kill kinetics studies as dramatically as it did the MBC results. Thus, bactericidal testing results are markedly influenced by the technique employed, and further in vivo studies are necessary to fully evaluate the efficiency of oxacillin against beta-lactamase-hyperproducing strains of S. aureus.

Role of the cell envelope in bacterial adaptation to growth in vivo in infections

To establish an infection, a pathogenic bacterium must adapt to growth in the hostile environment encountered in vivo in host tissues. The cell envelope plays a crucial role in this adaptive process, since it is involved in promoting adhesion to and colonisation of host tissues, in the acquisition of essential nutrients and in conferring resistance to host defences and to antibiotics. Its properties are ultimately determined by the information stored within the genome, which also contains the potential to respond to environmental change. The macromolecular structure and function of the cell envelope are largely determined by the growth environment and, in particular, specific nutrient limitation, growth rate, growth temperature and replication in suspension or within a surface-associated biofilm. Bacteria growing in vivo will manufacture envelopes characteristic of that environment and which will differ markedly in physiology, biochemistry and immunogenicity from those of cells grown in a standard laboratory medium. In vivo, the ability to withhold iron is an important component of the host's defence and iron deprivation has a pronounced effect on the metabolism and cell envelope properties of pathogenic bacteria. The phenotypic plasticity of the bacterial cell surface plays an important role in determining susceptibility to host defences and antibiotics and has important implications for the design and evaluation of new therapeutic strategies for the treatment and prevention of bacterial infections.

Oxacillin-induced inhibition of protein and RNA synthesis in a tolerant Staphylococcus aureus isolate

A clinical isolate of Staphylococcus aureus was found to be tolerant (MBC much greater than MIC) to a number of beta-lactam antibiotics, including oxacillin. Biophotometric analysis showed that a number of concentrations of oxacillin were capable of stimulating rapid cellular lysis in this organism, but the extent of lysis was antibiotic concentration dependent and limited. Cell cultures treated with an antibiotic concentration yielding the maximum rate and extent of lysis were analyzed for protein and RNA synthesis by pulse-labeling techniques. RNA synthesis was initially stimulated and then severely inhibited. Protein synthesis was not inhibited initially; however, the increase in the rate of synthesis expected as the result of logarithmic growth was not observed. Instead, the antibiotic-treated culture maintained for approximately 50 min the rate of protein synthesis ongoing at the time of antibiotic addition. The rate of protein synthesis declined thereafter. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of protein samples taken 1 and 3 h after antibiotic addition showed that the shutdown of protein synthesis was not coordinate but rather was suggestive of the operation of a stress regulon perhaps similar to those responsible for heat shock, SOS, and oxidation stress.