Analysis of trends in antimicrobial resistance patterns among clinical isolates of Bacteroides fragilis group species from 1990 to 1994

Identification and Antimicrobial Susceptibility Testing of Anaerobic Bacteria: Rubik's Cube of Clinical Microbiology?

Anaerobic bacteria have pivotal roles in the microbiota of humans and they are significant infectious agents involved in many pathological processes, both in immunocompetent and immunocompromised individuals. Their isolation, cultivation and correct identification differs significantly from the workup of aerobic species, although the use of new technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, whole genome sequencing) changed anaerobic diagnostics dramatically. In the past, antimicrobial susceptibility of these microorganisms showed predictable patterns and empirical therapy could be safely administered but recently a steady and clear increase in the resistance for several important drugs (β-lactams, clindamycin) has been observed worldwide. For this reason, antimicrobial susceptibility testing of anaerobic isolates for surveillance purposes or otherwise is of paramount importance but the availability of these testing methods is usually limited. In this present review, our aim was to give an overview of the methods currently available for the identification (using phenotypic characteristics, biochemical testing, gas-liquid chromatography, MALDI-TOF MS and WGS) and antimicrobial susceptibility testing (agar dilution, broth microdilution, disk diffusion, gradient tests, automated systems, phenotypic and molecular resistance detection techniques) of anaerobes, when should these methods be used and what are the recent developments in resistance patterns of anaerobic bacteria.

Risk Factors for Resistance to β-Lactam/β-Lactamase Inhibitors and Ertapenem in Bacteroides Bacteremia

The objective of this study was to determine risk factors for the development of resistance to β-lactams/β-lactamase inhibitors (βL/βLIs) and ertapenem among Bacteroides species bacteremia. We conducted a retrospective case-control study of 101 adult patients with Bacteroides species bacteremia at a 1,051-bed tertiary care medical center. The duration of exposure to βL/βLIs (odds ratio [OR], 1.25; 95% confidence interval [CI], 1.08 to 2.31) was the only independent risk factor for resistance.

Diagnosis and management of complicated intra-abdominal infection in adults and children: guidelines by the Surgical Infection Society and the Infectious Diseases Society of America

Evidence-based guidelines for managing patients with intra-abdominal infection were prepared by an Expert Panel of the Surgical Infection Society and the Infectious Diseases Society of America. These updated guidelines replace those previously published in 2002 and 2003. The guidelines are intended for treating patients who either have these infections or may be at risk for them. New information, based on publications from the period 2003-2008, is incorporated into this guideline document. The panel has also added recommendations for managing intra-abdominal infection in children, particularly where such management differs from that of adults; for appendicitis in patients of all ages; and for necrotizing enterocolitis in neonates.

Diagnosis and Management of Complicated Intra-abdominal Infection in Adults and Children: Guidelines by the Surgical Infection Society and the Infectious Diseases Society of America

Evidence-based guidelines for managing patients with intra-abdominal infection were prepared by an Expert Panel of the Surgical Infection Society and the Infectious Diseases Society of America. These updated guidelines replace those previously published in 2002 and 2003. The guidelines are intended for treating patients who either have these infections or may be at risk for them. New information, based on publications from the period 2003–2008, is incorporated into this guideline document. The panel has also added recommendations for managing intra-abdominal infection in children, particularly where such management differs from that of adults; for appendicitis in patients of all ages; and for necrotizing enterocolitis in neonates.

In vitro susceptibility to selected antibiotics in bacteria of the Bacteroides fragilis group

Susceptibility of strains of Bacteroides fragilis group (1284 isolates from cancerous and noncancerous patients in 1994-2004) showed an increase in resistance toward some antibiotics (by 9 % toward penicillin and 8 % toward clindamycin) compared with the resistance level of 10 years ago. The increase in resistance was not detected in the case of ampicillin + sulbactam and metronidazole.

Characterization of beta-lactam-resistant Bacteroides fragilis isolates by use of PCR fingerprinting

PCR fingerprinting was used for characterization of 35 beta-lactam-resistant Bacteroides fragilis strains isolated in Sweden and Hungary. Ten B. fragilis strains showed unique PCR fingerprints by use of the M13 core primer. Their main product was a DNA fragment with a length of 2000-bp which was absent in the other 25 strains and the reference strain B. fragilis ATCC 25285. The 2000-bp fragment from four imipenem-resistant strains gave rise to positive reactions in a specific PCR for detection of ccrA. Printed by the T3B primer, five B. fragilis strains, including the imipenem-resistant strains showed unique PCR fingerprints. The investigated imipenem-resistant strains produced carbapenem-hydrolysing metallo-beta-lactamases. The study indicates that the unique PCR fingerprinting profiles shown in highly beta-lactam resistant B. fragilis strains are correlated to antimicrobial resistance. The PCR fingerprinting technique is a useful tool for differentiation of Bacteroides fragilis strains with high-level beta-lactam resistance.

Occurrence of enterotoxigenic and nonenterotoxigenic Bacteroides fragilis in calves and evaluation of their antimicrobial susceptibility

Bacteroides fragilis is considered an important clinical pathogen and the most common anaerobe isolated from human and animal clinical specimens; enterotoxigenic strains produce diarrhea. The presence of enterotoxigenic (ETBF) and nonenterotoxigenic B. fragilis in stool samples from calves with or without acute diarrhea and the antimicrobial susceptibility of the strains were evaluated. The stool samples were plated onto a selective B. fragilis–bile–esculin agar, and incubated anaerobically (10% CO₂/90% N₂), at 37°C, for 72 h. Species of the B. fragilis group were identified by using the API 32‐A kit. Enterotoxigenic strains were detected by PCR and the cytotoxic assay. From 54 diarrhea and 54 nondiarrhea stools, 124 and 92 members of the B. fragilis group, respectively, were recovered. Only two ETBF strains were isolated from two different diarrhea samples and the bft gene was detected in both. Moreover, the bft gene was detected in DNA from four different diarrheal stools samples but no ETBF strain was recovered. All the bacteria were susceptible to chloramphenicol, imipenem, moxifloxacin, piperacillin/tazobactam, metronidazole and tigecycline. Most of the isolates from both calves with and without diarrhea were resistant to all metals. Our results are of concern, and suggest the need to increase the surveillance of antibiotic and metal resistance of this microbial group isolated from animal production such as calves.

Isolation of metronidazole-resistant Bacteroides fragilis carrying the nimA nitroreductase gene from a patient in Washington State

Members of the Bacteroides fragilis group are among the most common anaerobic bacterial isolates in clinical specimens. Metronidazole, a 5-nitroimidazole, is often used as empirical therapy for anaerobic infections. Susceptibility testing is not routinely performed because of nearly universal susceptibility of Bacteroides spp. to this agent. We report a case of metronidazole-resistant Bacteroides fragilis in the United States and demonstrate the presence of the nimA gene, encoding a nitroreductase previously shown to mediate resistance to 5-nitroimidazole antimicrobial agents in B. fragilis strains from Europe and Africa. Because clinical failures in Bacteroides infections have been associated with the use of inactive antimicrobial agents, clinicians need to be aware of the possibility of metronidazole-resistant B. fragilis strains in the United States and the importance of susceptibility testing in selected situations.

Identification of Bacteroides thetaiotaomicron on the basis of an unexpected specific amplicon of universal 16S ribosomal DNA PCR

We applied a set of commonly used universal primers (primers RW01 and DG74) to amplify partial fragments of 16S ribosomal DNA for bacterial identification and found an unexpected amplicon (547 bp), in addition to the expected 362-bp product, in samples containing Bacteroides thetaiotaomicron. It was demonstrated that the internal sequence (508 bp, excluding the primers) of the 547-bp amplicon was identical to the genomic sequence from nucleotide positions 165800 to 166307 of B. thetaiotaomicron type strain VPI-5482 by a BLAST search of the sequences in the GenBank database. The existence of this unexpected yet specific amplicon strongly indicated the presence of B. thetaiotaomicron in the sample, and it was found that it could be used to discriminate B. thetaiotaomicron from closely related species. Another set of PCR primers specific for B. thetaiotaomicron was developed on the basis of the sequence of this 547-bp genomic fragment. Both PCR-based assays showed the same sensitivity (88%) and specificity (100%).

Virulence markers and antimicrobial susceptibility of bacteria of the Bacteroides fragilis group isolated from stool of children with diarrhea in São Paulo, Brazil

Bacteroides fragilis has been isolated from several human and non-human monomicrobial and mixed infections. In this study, some virulence markers and the antimicrobial susceptibility of bacteria of the B. fragilis group isolated from children's stools were evaluated. All the 64 isolates showed the following characteristics: capsulated, beta-hemolytic, hydrophilic, and serum-resistant. Only, 24 (37.5%) strains were resistant at 60 masculine C, for 30 min, and among them, 12 (18.75%) were resistant at 60 masculine C, for 60 min. Also, none strain was resistant at 100 masculine C. Four strains were able to hemagglutinate erythrocytes and D-mannose, D-galactose, D-arabinose, and D-xylose inhibited hemagglutination in 2 B. fragilis strains (p76a, p76b). The hemagglutination in the strain B. uniformis p3-2 was inhibited by D-xylose and D-galactose. The bft gene detection and the enterotoxin production were observed only in 13 EF-enterotoxigenic species. Fragilysin activity was confirmed on HT-29 cells. The antimicrobial determination confirmed that both imipenem and metronidazole were efficient against B. fragilis species; all the strains were resistant to lead and nickel. Plasmids of 2.9, 4.4, 4.8, and 8.9 kb were observed in 6 tested strains. These results show the values of the species identification from clinical infections, as well as of the periodic evaluation of the resistance patterns of the B. fragilis group at Brazilian medical institutions.

In vitro activities of tigecycline against the Bacteroides fragilis group

The in vitro activities of tigecycline were tested against 831 isolates of the Bacteroides fragilis group representing all of the species within the group. On a weight-to-weight basis (8 microg/ml), tigecycline was more active than clindamycin, minocycline, trovafloxacin, and cefoxitin and less active than imipenem or piperacillin-tazobactam against all isolates of the B. fragilis group. Tigecycline geometric mean MICs were statistically higher against B. distasonis than other Bacteroides species (P value of 0.0001).

Use of denaturing gradient gel electrophoresis for analysis of the stool microbiota of hospitalized patients

Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified ribosomal RNA gene amplicons was used to study the stool microbiota of hospitalized patients and to examine the effect of antibiotic therapy. For one patient, 16 anaerobic species identified by random cloning and sequencing of PCR-amplified rRNA genes from stool were represented by bands on the DGGE gel. DGGE analysis and similarity index comparisons demonstrated that the anaerobic microbiota of this individual remained stable in the absence of antibiotic therapy, was minimally affected by ciprofloxacin but markedly reduced by clindamycin therapy, and recovery of some organisms was evident within days after discontinuation of clindamycin. DGGE analysis of additional patients demonstrated similar disruptions of the intestinal microbiota associated with antibiotic therapy. The DGGE banding patterns of nine patients showed considerable variability, but several bands were shared among patients. Thus, our findings are consistent with previous studies that utilized culture techniques, and suggest that DGGE is a useful technique for analysis of the stool microbiota of hospitalized patients.

In vitro activities of newer quinolones against bacteroides group organisms

The activities of BMS-284576, clinafloxacin, moxifloxacin, sitafloxacin, trovafloxacin, imipenem, cefoxitin, and clindamycin against 589 Bacteroides fragilis group isolates were determined. The activity of BMS-284576 was comparable to that of trovafloxacin. Sitafloxacin and clinafloxacin were the most active quinolones, and moxifloxacin was the least active. B. fragilis was the most susceptible of the species, and Bacteroides vulgatus was the most resistant. Association of specific antibiotic resistance with Bacteroides species was noted for all quinolones.

High incidence of cefoxitin and clindamycin resistance among anaerobes in Taiwan

Susceptibilities to 16 antimicrobial agents were determined by measurement of MICs for 344 isolates of anaerobic bacteria recovered from patients with significant infections. Resistance rates varied among antimicrobial agents and the species tested. The beta-lactams were more active in gram-positive than in gram-negative anaerobes. Resistance to meropenem was low (<1%). For beta-lactam-beta-lactamase inhibitors, piperacillin-tazobactam was most active for all species (resistance, <6%). The rates of resistance to cefoxitin (31 to 65%) and clindamycin (50 to 70%) for non-Bacteroides fragilis species of the B. fragilis group were higher than those for B. fragilis (4% resistant to cefoxitin and 33% resistant to clindamycin). Among members of B. fragilis group, Bacteroides thetaiotaomicron was the most resistant to clindamycin (70%) and cefoxitin (65%). Rates of susceptibility to imipenem and metronidazole for B. fragilis continue to be high compared to those from a previous study 10 years ago. However, resistance to metronidazole was found recently in five strains of B. fragilis. We analyzed the genetic relationships among the metronidazole-resistant B. fragilis strains by pulsed-field gel electrophoresis. The metronidazole-resistant B. fragilis strains showed genotypic heterogeneity, excluding the dissemination of a single clone.

Multilaboratory comparison of anaerobe susceptibility results using 3 different agar media

A 5-laboratory study was performed that used the National Committee for Clinical Laboratory Standards (NCCLS) reference agar dilution method with 3 media formulations to determine whether the use of different media would affect minimum inhibitory concentration (MIC) results. Wilkins-Chalgren, Brucella-based blood agar (BRU), and Wilkins-Chalgren agar plus blood (WCB) and 6 antibiotics (clindamycin, cefoxitin, ceftizoxime, piperacillin, metronidazole, and trovafloxacin) were evaluated with 58 isolates. The MIC values were compared, and a significant correlation of >0.80 was demonstrated for all media and each antibiotic/organism group. The cumulative rate of errors for all antibiotics was 0.1%. These data indicate that a change in the NCCLS reference medium for testing of anaerobic bacteria susceptibility to either BRU or WCB will not affect the MIC results for the antibiotics and organisms evaluated.

National survey on the susceptibility of Bacteroides Fragilis Group: report and analysis of trends for 1997-2000

The results of a multicenter US survey using the National Committee for Clinical Laboratory Standards currently recommended methodology for measuring in vitro susceptibility of 2673 isolates of Bacteroides fragilis group species were compared from 1997 to 2000. The test panel consisted of 14 antibiotics: 3 carbapenems, 3 beta-lactam-beta-lactamase inhibitors, 3 cephamycins, 2 fluoroquinolones, clindamycin, chloramphenicol, and metronidazole. Declines in the geometric mean minimum inhibitory concentrations were seen with imipenem, meropenem, ampicillin-sulbactam, and the cephamycins. Increased geometric means were observed with the fluoroquinolones and were usually accompanied by an increase in resistance rates. Bacteroides distasonis shows the highest resistance rates among beta-lactam antibiotics, whereas Bacteroides vulgatus shows the highest resistance levels among fluoroquinolones. B. fragilis shows the lowest resistance rates for all antibiotics. All strains were susceptible to chloramphenicol and metronidazole concentrations <8 microgram/mL. The data underscore the need for species identification and continued surveillance to monitor resistance patterns.

Intra-abdominal anaerobic infections: bacteriology and therapeutic potential of newer antimicrobial carbapenem, fluoroquinolone, and desfluoroquinolone therapeutic agents

Intra-abdominal infections are biphasic, synergistic processes with early peritonitis and bacteremia due to aerobes and a later abscess component due to anaerobes. Although Bacteroides fragilis is the most commonly recognized pathogen, other anaerobes, including other members of the B. fragilis-group species, are major components of infection. Anaerobic bacteremia is often associated with an intra-abdominal source. New antimicrobial agents with anaerobic activity are in various stages of development for the therapy of intra-abdominal infections. The in vitro activity and the currently available sparse clinical data are reviewed for a new carbapenem (ertapenem), several fluoroquinolones (trovafloxacin, moxifloxacin, and gemifloxacin), and a desfluoroquinolone (BMS-284756).

Susceptibility testing. Phenotypic and genotypic tests for bacteria and mycobacteria

Genotypic-based methods hold promise for the rapid and accurate detection or confirmation of antimicrobial resistance; however, phenotypic methods will continue to have an advantage when resistance to the same antimicrobial agent may be caused by several different mechanisms. The diversity of genetic mechanisms may exceed the capabilities of current molecular technology. Genotypic assays have the ability to detect resistance but not susceptibility. Although resutls can be obtained rapidly, many molecular methods are labor-intensive, expensive, and lack standardization. Clinical studies will be required to validate the genotypic approach to detection of antimicrobial resistance. Molecular assays are also at risk for false-positive results because of contamination of specimens by other specimens that carry the DNA targeted for the assay, or carryover of amplified target DNA (amplicons) from a previous PCR assay during sample preparation. Detection of certain genetic resistance loci in clinical specimens must be interpreted with caution, because organisms in normal flora may also harbor the same loci. All these factors must be taken into consideration when introducing a genotypic method in the clinical laboratory. Other considerations include cost, turnaround time, and assay performance. It must be emphasized that the bedside assessment of the patient should always be considered in addition to the results of antimicrobial susceptibility tests (whether phenotypic or genotypic) so that the best outcome is assured for the patient.

Antimicrobial resistance and clinical outcome of Bacteroides bacteremia: findings of a multicenter prospective observational trial

There is debate regarding the correlation between in vitro susceptibility testing and clinical response to therapy for Bacteroides bacteremia. We conducted a prospective multicenter observational study of 128 patients with bacteroides bacteremia. Outcome was correlated with results of in vitro susceptibility testing of Bacteroides isolates recovered from blood and/or nonblood sites, determined with use of 3 end points: mortality at 30 days, clinical response (cure vs. failure), and microbiological response (eradication vs. persistence). The mortality rate among patients who received inactive therapy (45%) was higher than among patients who received active therapy (16%; P=.04). Clinical failure (82%) and microbiological persistence (42%) were higher for patients who received inactive therapy than for patients who received active therapy (22% and 12%, respectively; P=.0002 and.06, respectively). In vitro activity of agents directed at Bacteroides species reliably predicts outcome: the specificity was 97%, and positive predictive value was 82%. Antimicrobial susceptibility testing may be indicated for patients whose blood specimens yield Bacteroides species.

Antimicrobial susceptibility testing: special needs for fastidious organisms and difficult-to-detect resistance mechanisms

Clinical microbiology laboratories are faced with the challenge of accurately detecting emerging antibiotic resistance among a number of bacterial pathogens. In recent years, vancomycin resistance among enterococci has become prevalent, as has penicillin resistance and multidrug resistance in pneumococci. More recently, strains of methicillin-resistant Staphylococcus aureus with reduced susceptibility to vancomycin have been encountered. In addition, molecular techniques have demonstrated that there are still problems detecting methicillin resistance in staphylococci, especially in coagulase-negative species. Among members of the family Enterobacteriaceae, mutated beta-lactamase enzymes may confer difficult-to-detect resistance to later-generation penicillins and cephalosporins. Anaerobic bacteria are no longer entirely predictable in their susceptibility to agents that might be selected for empiric therapy. Therefore, clinical microbiology laboratories may not be able to rely on a single susceptibility testing method or system to detect all those emerging resistant or fastidious organisms. For reliable detection, laboratories may need to employ conventional, quantitative susceptibility testing methods or use specially developed, single concentration agar screening tests for some resistant species. Certain of these screening tests are highly specific, while others may require additional confirmatory testing for definitive results. Therefore, laboratories must retain the versatility to apply several different approaches to detect resistance in both common and infrequently encountered bacterial pathogens.

PCR assay for species-specific identification of Bacteroides thetaiotaomicron

Bacteroides thetaiotaomicron is the second most frequently encountered species of the anaerobes isolated from clinical specimens. We developed a PCR-based assay for the rapid identification of B. thetaiotaomicron. Specific primers were based on shared amplicons of about 1.2 kb generated from B. thetaiotaomicron by randomly amplified polymorphic DNA. This 1.2-kb fragment was sequenced and then used to design a set of PCR amplification primers. This PCR generated an amplification product of 721 bp, which was unique to all 65 isolates of B. thetaiotaomicron tested. There was no amplification with isolates of other bacterial species. Restriction enzyme digestion of the amplification product and dot blot hybridization further verified the specificity of the assay. These results suggest that this PCR assay targets a nucleotide sequence that is strongly conserved in B. thetaiotaomicron. This simple and rapid PCR assay provides a rapid and accurate method for identification of B. thetaiotaomicron and shows promise for the detection of B. thetaiotaomicron in clinical samples.

Trends in antimicrobial resistance among clinical isolates of the Bacteroides fragilis group from 1992 to 1997 in Montreal, Canada

The objective of the present study was to analyze the susceptibility profiles of 911 clinical strains of the Bacteroides fragilis group isolated from 1992 to 1997 in our institution in order to monitor susceptibility changes over time. Whereas the rates of resistance to metronidazole, imipenem, piperacillin-tazobactam, ticarcillin-clavulanic acid, penicillin, piperacillin, and cefoxitin remained essentially unchanged, there was a significant increase in the rates of resistance to clindamycin, which rose from 8.2% in 1992 to 19.7% in 1997 (P < 0.0004).

Multicenter study of in vitro susceptibility of the Bacteroides fragilis group, 1995 to 1996, with comparison of resistance trends from 1990 to 1996

Antimicrobial resistance, including plasmid-mediated resistance, among the species of the Bacteroides fragilis group is well documented. An analysis of the in vitro susceptibility of B. fragilis group species referred between 1995 and 1996 as well as during a 7-year (1990 to 1996), prospective, multicenter survey of over 4,000 clinical isolates of B. fragilis group species was undertaken to review trends in the percent resistance to and geometric mean MICs of the antibiotics tested. There was a trend toward a decrease in the geometric mean MICs of most beta-lactam antibiotics, while the percent resistance to most agents was less affected. Within the species B. fragilis, the geometric mean MICs showed significant (P < 0.05) decreases for piperacillin-tazobactam, ticarcillin-clavulanate, piperacillin, ticarcillin, ceftizoxime, cefotetan, and cefmetazole; a significant increase was observed for clindamycin and cefoxitin. For the non-B. fragilis species, a significant decrease in the geometric mean MICs was observed for meropenem, ampicillin-sulbactam, ticarcillin-clavulanate, piperacillin, ticarcillin, ceftizoxime, and cefmetazole; a significant increase was observed for cefoxitin. Significant increases in percent resistance were observed within the B. fragilis strains for ticarcillin and ceftizoxime and within the non-B. fragilis isolates for cefotetan. Significant increases in percent resistance among all B. fragilis group species were observed for clindamycin, while imipenem showed no significant change in resistance trends. The trend analysis for trovafloxacin was limited to 3 years, since the quinolone was tested only in 1994, 1995, and 1996. During the 7 years analyzed, there was no resistance to metronidazole or chloramphenicol observed. The data demonstrate that resistance among the B. fragilis group species has decreased in the past several years, the major exception being clindamycin. The majority of the resistance decrease has been for the beta-lactams in B. fragilis, compared to other species. The reasons for these changes are not readily apparent.

Activities of new antimicrobial agents (trovafloxacin, moxifloxacin, sanfetrinem, and quinupristin-dalfopristin) against Bacteroides fragilis group: comparison with the activities of 14 other agents

The antimicrobial activities of trovafloxacin, moxifloxacin, sanfetrinem, quinupristin-dalfopristin, and 14 other antimicrobial agents against 218 Bacteroides fragilis group strains were determined. A group of 10 imipenem-resistant strains were also tested. Imipenem, meropenem, and sanfetrinem had the lowest MICs of all of the beta-lactams. Quinupristin-dalfopristin inhibited all of the strains at 2 microg/ml. Overall, the MICs of trovafloxacin and moxifloxacin for 90% of the strains tested were 1 and 2 microg/ml, respectively.

Clindamycin, metronidazole, and chloramphenicol

Clindamycin, metronidazole, and chloramphenicol are three antimicrobial agents useful in the treatment of anaerobic infections. Clindamycin is effective in the treatment of most infections involving anaerobes and gram-positive cocci, but emerging resistance has become a problem in some clinical settings. Metronidazole is effective in the treatment of infections involving gram-negative anaerobes, but it is unreliable in the treatment of gram-positive anaerobic infections and is ineffective in treating aerobic infections. Additionally, metronidazole is often the drug of choice in treating infections in which Bacteroides fragilis is a serious concern. Chloramphenicol is effective in the treatment of a wide variety of bacterial infections, including serious anaerobic infections, but is rarely used in Western countries because of concerns about toxicity, including aplastic anemia and gray baby syndrome.

Piperacillin/tazobactam: an updated review of its use in the treatment of bacterial infections

Piperacillin/tazobactam is a beta-lactam/beta-lactamase inhibitor combination with a broad spectrum of antibacterial activity encompassing most Gram-positive and Gram-negative aerobic bacteria and anaerobic bacteria, including many pathogens producing beta-lactamases. Evidence from clinical trials in adults has shown that piperacillin/tazobactam, administered in an 8:1 ratio, is an effective treatment for patients with lower respiratory tract, intra-abdominal, urinary tract, gynaecological and skin/soft tissue infections, and for fever in patients with neutropenia. Combination regimens of piperacillin/tazobactam plus an aminoglycoside are used to treat patients with severe nosocomial (hospital-acquired) infections. In clinical trials, piperacillin/tazobactam was significantly more effective than ticarcillin/clavulanic acid in terms of clinical and microbiological outcome in patients with community-acquired pneumonia. In patients with intra-abdominal infections, clinical and bacteriological response rates were significantly higher with piperacillin/tazobactam than with imipenem/cilastatin (administered at a dosage lower than is recommended in countries outside Scandinavia). Piperacillin/tazobactam in combination with amikacin was at least as effective as ceftazidime plus amikacin in the treatment of ventilator-associated pneumonia and was significantly more effective than ceftazidime plus amikacin in the empirical treatment of febrile episodes in patients with neutropenia or granulocytopenia. In other trials, the efficacy of piperacillin/tazobactam was similar to that of standard aminoglycoside-containing and other treatment regimens in patients with intra-abdominal, skin/soft tissue or gynaecological infections. Piperacillin/tazobactam is generally well tolerated. The most frequent adverse events are gastrointestinal symptoms (most commonly diarrhoea) and skin reactions. The incidence of adverse events with piperacillin/tazobactam is higher when the combination is given in combination with an aminoglycoside than when given as monotherapy.

CONCLUSION

Because of the broad spectrum of antibacterial activity provided by piperacillin/tazobactam, it is useful for the treatment of patients with polymicrobial infections caused by aerobic or anaerobic beta-lactamase-producing bacteria. Piperacillin/tazobactam appears to have a particularly useful role in the treatment of patients with intra-abdominal infections and, in combination with amikacin, in the treatment of patients with febrile neutropenia, especially given the current prevalence of Gram-positive infections in this group.

Carbapenems and monobactams: imipenem, meropenem, and aztreonam

Imipenem and meropenem, members of the carbapenem class of beta-lactam antibiotics, are among the most broadly active antibiotics available for systemic use in humans. They are active against streptococci, methicillin-sensitive staphylococci, Neisseria, Haemophilus, anaerobes, and the common aerobic gram-negative nosocomial pathogens including Pseudomonas. Resistance to imipenem and meropenem may emerge during treatment of P. aeruginosa infections, as has occurred with other beta-lactam agents; Stenotrophomonas maltophilia is typically resistant to both imipenem and meropenem. Like the penicillins, the carbapenems have inhibitory activity against enterococci. In general, the in vitro activity of imipenem against aerobic gram-positive cocci is somewhat greater than that of meropenem, whereas the in vitro activity of meropenem against aerobic gram-negative bacilli is somewhat greater than that of imipenem. Daily dosages may range from 0.5 to 1 g every 6 to 8 hours in patients with normal renal function; the daily dose of meropenem, however, can be safely increased to 6 g. Infusion-related nausea and vomiting, as well as seizures, which have been the main toxic effects of imipenem, occur no more frequently during treatment with meropenem than during treatment with other beta-lactam antibiotics. The carbapenems should be considered for treatment of mixed bacterial infections and aerobic gram-negative bacteria that are not susceptible to other beta-lactam agents. Indiscriminate use of these drugs will promote resistance to them. Aztreonam, the first marketed monobactam, has activity against most aerobic gram-negative bacilli including P. aeruginosa. The drug is not nephrotoxic, is weakly immunogenic, and has not been associated with disorders of coagulation. Aztreonam may be administered intramuscularly or intravenously; the primary route of elimination is urinary excretion. In patients with normal renal function, the recommended dosing interval is every 8 hours. Patients with renal impairment require dosage adjustment. Aztreonam is used primarily as an alternative to aminoglycosides and for the treatment of aerobic gram-negative infections. It is often used in combination therapy for mixed aerobic and anaerobic infections. Approved indications for its use include infections of the urinary tract or lower respiratory tract, intra-abdominal and gynecologic infections, septicemia, and cutaneous infections caused by susceptible organisms. Concurrent initial therapy with other antimicrobial agents is recommended before the causative organism has been determined in patients who are seriously ill or at risk for gram-positive or anaerobic infection.

Effect of pH on in vitro antimicrobial susceptibility of the Bacteroides fragilis group

We studied the effect of pH (7.1, 6.3, and 5.8) on the in vitro susceptibilities of 59 isolates of Bacteroides fragilis and 60 isolates of other B. fragilis group species to trovafloxacin, ciprofloxacin, clindamycin, ampicillin-sulbactam, piperacillin-tazobactam, imipenem, and meropenem. For each agent tested the geometric mean MIC was highest at pH 5.8, intermediate at pH 6.3, and lowest at pH 7.1. The magnitude of the pH effect varied greatly among different antibiotics. These data show that an acidic pH decreases the in vitro susceptibilities of the B. fragilis group to several antibiotics.