Surveillance and trends of antimicrobial resistance among clinical isolates of anaerobes in Kuwait hospitals from 2002 to 2007

The susceptibility trends for all anaerobes processed by the Anaerobe Reference Laboratory against various antibiotics were determined by using data for 2557 isolates referred by all government hospitals in Kuwait from 2002 to 2007. MIC were determined for the following anti-anaerobic antibiotics: amoxicillin–clavulanic acid, clindamycin, imipenem, meropenem, metronidazole, penicillin, piperacillin, piperacillin–tazobactam and vancomycin (for Gram-positive anaerobes only), using E-test method. The commonest isolates were Bacteroides fragilis (36.8%), followed by Peptostreptococcus spp. (21.9%), Bacteroides ovatus (15.5%) and Prevotella bivia (12.1%). In addition, Prevotella oralis and other Bacteroides spp. represented 8.5% and 8.1% of total number of isolates, respectively. Resistance rate varied among the antimicrobial agents and the species tested. The β-lactams, with the exception of penicillin, were the most active drugs. Piperacillin–tazobactam was the only antimicrobial agent to which all the isolates were uniformly susceptible. Imipenem and metronidazole were highly active with resistance rate of only <5% recorded against most isolates. However, 42.8, 55.8 and 9.3% of Clostridium difficile isolates were resistant to imipenem, clindamycin and meropenem, respectively. It is noteworthy that from 2002 to 2007, there was a gradual increase in resistance rates to clindamycin, amoxicillin–clavulanic acid and piperacillin among B. fragilis. Periodic surveillance of antibiotic resistance among the anaerobic bacteria is recommended as a guide to empiric antibiotic use and formulation of guideline for appropriate choice of antimicrobial therapy in anaerobic infections.     

Antibiotic Resistance Among Anaerobes: What Does it Mean?

Antibiotic resistance among anaerobes is increasing, with significant resistance to clindamycin, cephalosporins, cephamycins, and penicillins noted at community hospitals and major medical centers. A total of 615 anaerobes isolated from various Chicago area hospitals in 1996 were tested against 13 antibiotics, and the resistance patterns compared with similar data from 1991. For the Bacteroides fragilis group anaerobes, the most effective antibiotics were the β-lactam/β-lactamase inhibitor combination agents, carbapenems, trovafloxacin and metronidazole. High levels of resistance to clindamycin, piperacillin, cefoxitin and ceftizoxime were seen 1996. For non- B. fragilis group anaerobes, resistance was mush lower, and was notable only in Clostridium spp. (clindamycin and cephamycins) and Prevotella spp. (clindamycin and piperacillin). Despite the prevalence of antibiotic resistance among anaerobes, the frequency of antimicrobial susceptibility testing of anaerobes is declining. There are a number of factors that account for this decline, including a general reduction in funding of hospital clinical laboratories, a concomitant loss of expertise at these institutions, a lack of automated testing for anaerobes, and a failure to consider resistance as important to clinicians. The case for increased susceptibility testing is built upon the changing patterns of resistance such as those reported in this paper, the identification and transfer of genetic determinants corresponding to antibiotic resistance, as well as the correlation of resistance and clinical outcome.     

Update on resistance of Bacteroides fragilis group and related species with special attention to carbapenems 2006-2009

The susceptibility trends for the species of the Bacteroides fragilis group against various antibiotics were determined using data from 4 years [2006-2009] on 1957 isolates referred by 8 medical centers participating in a National Survey for the Susceptibility of B. fragilis. The antibiotic test panel included doripenem, ertapenem, imipenem, meropenem, ampicillin:sulbactam, piperacillin:tazobactam, cefoxitin, clindamycin, moxifloxacin, tigecycline, chloramphenicol and metronidazole. MICs were determined using agar dilution methods following CLSI recommendations. Genetic analysis of isolates from 2008 with elevated MICs (>2 μg/mL) to one or more of the carbapenems to detect presence of the cfiA gene was performed using PCR methodology. The results showed an increase in the resistance rates to the β-lactam antibiotics. High resistance rates were seen for clindamycin and moxifloxacin (as high as 60% for clindamycin and >80% for moxifloxacin), with relatively stable low resistance (5.4%) for tigecycline. For carbapenems, resistance in B. fragilis was 1.1%-2.5% in 2008-9. One isolate resistant to metronidazole (MIC 32 μg/mL) was observed as well as isolates with elevated MICs to chloramphenicol (16 μg/mL). Genetic analysis indicated that the cfiA gene was present in some but not all of the isolates with high MICs to the carbapenems. These data indicate that there continue to be changes in susceptibility over time, and that resistance can be seen among the carbapenems. High antibiotic resistance rates tend to be associated with specific species.     

Homology between clindamycin resistance plasmids in Bacteroides

Two different species of clindamycin-resistant Bacteroides were isolated from the same infection. One isolate contained a single 15-kb plasmid (pCP1) which encoded transferable clindamycin resistance. pCP1 appears similar to the Bacteroides clindamycin resistance plasmid pBFTM10 isolated independently by F.P. Tally, D.R. Snydman, M.J. Shimell, and M.H. Malamy (1982, J. Bacteriol. 151, 686-691). The second strain had a 10-kb plasmid (pCP2) but did not transfer resistance. DNA hybridization studies revealed that pCP1 shares a 5-kb region of homology with the B. fragilis R plasmid pBF4 studied by R.A. Welch and F.L. Macrina (1981, J. Bacteriol. 145, 867-872). This region in both plasmids was shown to be bounded by homologous direct repeats and contains the putative clindamycin resistance determinant. pCP1 and pCP2 were found to share extensive homology but sequences homologous to the clindamycin resistance region were missing from pCP2 and found instead in the whole cell DNA of the host strain. These results identify a transposon-like structure on Bacteroides clindamycin resistance plasmids.     

Antibiotic resistance among anaerobic Gram-negative bacilli: lessons from a French multicentric survey

Temporal changes of antibiotic susceptibilities among anaerobes in France are followed in our laboratory since 1992. For Bacteroides strains, resistance increased from 1992 to 1998 for amoxicillin-clavulanic acid, cefotetan and clindamycin. The present study evaluates the situation in 2000 for 434 Gram-negative anaerobic clinical isolates (obtained from 9 large university hospitals) by testing amoxicillin and ticarcillin alone or combined with clavulanic acid, cefoxitin, cefotetan, imipenem, clindamycin and metronidazole (using the NCCLS-approved method for MIC determination. The main genera tested included Bacteroides (359 strains of the fragilis group), Prevotella (40 strains), Fusobacterium (23 strains) and miscellaneous species (8 strains). Resistance rates within the B. fragilis group were: amoxicillin-clavulanic acid 5.6%, ticarcillin 33%, ticarcillin-clavulanic acid 2%, cefoxitin 13%, cefotetan 44%, clindamycin 33%, imipenem 1% and metronidazole <1%, respectively. Only one strain of B. fragilis was resistant to metronidazole (MIC=64 mg/L); due to the presence of the nimA gene on the chromosome. Resistance to imipenem or metronidazole was only found among the B. fragilis species. These two former drugs excepted, B. fragilis was less resistant to antibiotics than the other species. beta-lactamase production was detected for 357/359 strains of the fragilis group, 26/40 stains of Prevotella and 3/23 strains of Fusobacterium. Dynamic changes of antibacterial resistance are occurring within the B. fragilis group: decreased resistance to amoxicillin-clavulanic acid, ticarcillin-clavulanic acid, imipenem while resistance for cefoxitin, cefotetan, clindamycin continues to increase. Regular antibiotic surveys are needed as a source of information to guide the empirical therapy of anaerobic infections.     

Clindamycin plus gentamicin as expectant therapy for presumed mixed infections.

The prevalence of obligate anaerobes was studied prospectively in 60 patients with severe sepsis of intra-abdominal, soft tissue, female genital or oropulmonary origin. In addition, the efficacy of clindamycin (for anaerobes) plus gentamicin (for aerobic bacteria, especially coliforms) as initial empiric therapy in these patients was evaluated. Among 54 patients with cultural proof of infection, anaerobic pathogens were recovered from 52%. Nineteen patients had bacteremia; Bacteroides fragilis and Klebsiella pneumoniae were the most prevalent pathogens, being isolated in five patients each. Infection was eradicated in 56 of the 60 patients (93%). Mortality related to sepsis was 7% in the entire group, 16% in patients with bacteremia and 2% in patients without bacteremia. Eighty-five percent of aerobic isolates tested were susceptible in vitro to either gentamicin or clindamycin; 97% of anaerobic isolates were inhibited by 5 mug/ml of clindamycin.     

Transferable lincosamide-macrolide resistance in Bacteroides.

Inter- and intraspecies transfer of resistance to clindamycin, lincomycin, and erythromycin in the strict anaerobe, Bacteroides, is described. This lincosamide-macrolide resistance was found to be specified by a 27 × 10⁶-dalton plasmid, designated pBF4, originally identified in a clinical Bacteroides fragilis isolate. Transfer of this plasmid to a strain of Bacteroides uniformis was demonstrated to occur by a deoxyribonuclease insensitive process which required cell-to-cell contact. Chloroform sterilized donor cell supernatants or filtrates of donor cells did not mediate resistance transfer. Transfer of the antibiotic resistance and pBF4 plasmid deoxyribonucleic acid (DNA) were always coincident. Drug resistant progeny recovered from such matings were able to transfer the pBF4 plasmid and its associated resistance markers to a suitable B.fragilis recipient strain. Compared to interspecies matings, resistance transfer was 100- to 1000-fold greater between isogenic donor and recipient strains, suggesting the possibility of a host controlled restriction-modification system.     

Antibiotic susceptibility of clinically relevant anaerobes in Estonia from 1999 to 2001

At present little or no data is available regarding the resistance profiles of anaerobic bacteria in relation to the general usage of antibiotics. The objective of this study was to assess whether any potential relationship exists between the dynamics of antibiotic resistance of anaerobic bacteria and the consumption of antibiotics during the last 3 years within the Estonian population. In total, 416 anaerobic isolates were investigated from various clinical samples. The anaerobes were isolated on Wilkins-Chalgren Agar, incubated in an anaerobic glove box and identified by standard methods. beta-lactamase negative strains were tested against metronidazole, clindamycin, benzylpenicillin and the positive strains were further tested against metronidazole, clindamycin, and ampicillin/sulbactam by E-tests. The results of the susceptibility tests were interpreted according to the current criteria of NCCLS. Data from the Estonian State Agency of Medicines was used to assess the antibiotic consumption rate in the population (Defined Daily Doses per 1000 inhabitants annually). The following species of anaerobes were isolated: B. fragilis group, Bacteroides sp., Fusobacterium sp., Porphyromonas sp., Prevotella sp., Peptostreptococcus sp., in addition to various unidentified Gram-positive rods. Metronidazole resistance was not found among Gram-negative bacteria despite a relatively high consumption of this antimicrobial agent in Estonia. Only ampicillin/sulbactam demonstrated excellent in vitro activity against all anaerobes. Unexpectedly despite a relatively low rate of consumption of clindamycin a high rate of resistance to this agent occurred; a similar situation was noted for penicillin. In the present study we did not observe a relationship between the changes in antibiotic consumption (DDD/1000) rate and the resistance pattern of anaerobic bacteria to metronidazole, clindamycin, penicillin and ampicillin/sulbactam during a 3-year follow-up period. High resistance to penicillin among some species and also to clindamycin is similar to the global trend and argues for limited use of these antibiotics in empirical treatment. We would suggest that monitoring of local susceptibility pattern is necessary for the selection of initial empirical therapy.     

Chemotherapy of bacteroides infection under clinical and experimental conditions

Bacteroides were detected in pus and bioptates of the majority of 125 patients with purulent and purulent septic infections of various localization. In 90 per cent of the subjects the bacteroides were detected in association with aerobes, facultative and obligate anaerobic bacteria. The species of the bacteroides and concomitant microflora isolated from the cases with different diseases were defined by endogenic sources of the microbial contamination. Sensitivity of 112 bacteroide strains to antimicrobial agents was tested. Chloramphenicol, clindamycin, metronidazole, cefotaxime and heliomycin proved to be the most efficient. Efficacy of heliomycin was detected in hamsters with experimental bacteroide infection in the buccal sac. Thorough bacteriological examination is required for rational chemotherapy of bacteroide infections.     

Inference of Network Dynamics and Metabolic Interactions in the Gut Microbiome

We present a novel methodology to construct a Boolean dynamic model from time series metagenomic information and integrate this modeling with genome-scale metabolic network reconstructions to identify metabolic underpinnings for microbial interactions. We apply this in the context of a critical health issue: clindamycin antibiotic treatment and opportunistic Clostridium difficile infection. Our model recapitulates known dynamics of clindamycin antibiotic treatment and C. difficile infection and predicts therapeutic probiotic interventions to suppress C. difficile infection. Genome-scale metabolic network reconstructions reveal metabolic differences between community members and are used to explore the role of metabolism in the observed microbial interactions. In vitro experimental data validate a key result of our computational model, that B. intestinihominis can in fact slow C. difficile growth.     

Susceptibility testing of anaerobic bacteria: A review of current methods and future prospects

The current NCCLS document, M11 A2, describes two methods for susceptibility testing of anaerobic bacteria. The reference method utilizes an agar dilution procedure, which is labor intensive and not convenient for testing individual patient isolates. The broth microdilution method does not support the growth of 15–40% clinical isolates and demonstrates poor correlation with the reference method for some members of the Bacteroides fragilis group with β-lactam agents and clindamycin. Etest is a new technique that incorporates an antibiotic gradient onto a plastic strip and utilizes agar media. This method is easily performed, permits growth of all anaerobes, and provides quantitative MICs for rapidly growing strains after overnight (20 hr) incubation. This method is convenient and reliable and enables the laboratory to provide the clinician with MIC data for individual patient isolates within a clinically relevant time period.     

Variability in pediatric infectious disease consultants' recommendations for management of community-acquired pneumonia

Background

Community-acquired pneumonia (CAP) is a common childhood infection. CAP complications, such as parapneumonic empyema (PPE), are increasing and are frequently caused by antibiotic-resistant organisms. No clinical guidelines currently exist for management of pediatric CAP and no published data exist about variations in antibiotic prescribing patterns. Our objectives were to describe variation in CAP clinical management for hospitalized children by pediatric infectious disease consultants and to examine associations between recommended antibiotic regimens and local antibiotic resistance levels.

Methods

We surveyed pediatric members of the Emerging Infections Network, which consists of 259 pediatric infectious disease physicians. Participants responded regarding their recommended empiric antibiotic regimens for hospitalized children with CAP with and without PPE and their recommendations for duration of therapy. Participants also provided information about the prevalence of penicillin non-susceptible S. pneumoniae and methicillin-resistant S. aureus (MRSA) in their community.

Results

We received 148 responses (57%). For uncomplicated CAP, respondents were divided between recommending beta-lactams alone (55%) versus beta-lactams in combination with another class (40%). For PPE, most recommended a combination of a beta-lactam plus an anti-MRSA agent, however, they were divided between clindamycin (44%) and vancomycin (57%). The relationship between reported antibiotic resistance and empiric regimen was mixed. We found no relationship between aminopenicillin use and prevalence of penicillin non-suscepetible S. pneumoniae or clindamycin use and clindamycin resistance, however, respondents were more likely to recommend an anti-MRSA agent when MRSA prevalence increased.

Conclusions

Substantial variability exists in recommendations for CAP management. Development of clinical guidelines via antimicrobial stewardship programs and dissemination of data about local antibiotic resistance patterns represent opportunities to improve care.     

Trends in antibiotic resistance in Prevotella species from patients of the University Hospital of Maxillofacial Surgery,Sofia, Bulgaria,in 2003–2009

Head-and-neck infections often involve anaerobes such as Prevotella species. Aim of the present study was to assess the evolution and the factors associated with resistance in Prevotella species to penicillin, clindamycin, metronidazole, tetracycline and β-lactams/β-lactamase inhibitors (BL/BLIs). In total, 192 Prevotella strains, isolated from patients with oral and head-and-neck infections, were evaluated. Common isolates were Prevotella intermedia and Prevotella melaninogenica within the pigmented species as well as Prevotella oris and Prevotella oralis group within the non-pigmented species. Overall resistance was 43.2% for penicillin, 10.9% for clindamycin, 0% for metronidazole. Nonsusceptibility to tetracycline was 29.1% without significant differences in resistance rates between pigmented and other species. Penicillin resistant strains were β-lactamase positive. From 2003–2004 to 2007–2009, penicillin resistance rates increased about four-fold (from 15.4% to 60.6%). Clindamycin resistance did not show evolution, whereas tetracycline nonsusceptibility decreased from 43.3% in 2003–2004 to 20.7% in 2007–2009. Except for one (0.5%) P. oralis strain with intermediate susceptibility to BL/BLIs, the other strains were susceptible to the agents. In conclusion, in Prevotella strains from patients with head-and-neck infections, the resistance rate to penicillin increased, that to clindamycin remained stable and the nonsusceptibility rate to tetracycline decreased during the period. Activity against >99% of Prevotella strains was observed with metronidazole and BL/BLIs. The penicillin resistance and tetracycline nonsusceptibility were associated with the year of study, national antibiotic consumption and possibly with previous treatment (for tetracycline). The evolution of penicillin resistance in Prevotella strains was highly dynamic.     

In vitro activity of 11 antibiotics against 74 anaerobes isolated from pediatric intra-abdominal infections

The in vitro activity of 11 antimicrobials was tested against 74 recent anaerobic isolates obtained from pretreatment cultures in pediatric patients with complicated intra-abdominal infections using the CLSI M11-A-6 agar dilution method. Carbapenems, beta-lactamase inhibitor combinations and metronidazole retained good activity, while all Bacteroides fragilis group species produced beta-lactamase and were penicillin resistant and 43% were either intermediately susceptible or resistant to clindamycin. Cefoxitin had moderate activity against B. fragilis but poor activity against Bacteroides thetaiotaomicron and other B. fragilis group isolates.     

Antibiotic susceptibility of Bacteroides fragilis group strains in Hungary

Resistance rates to different antibiotics of 495 Bacteroides fragilis group strains were followed between 1987 and 1994 in Hungary. In 1992 the strains were collected in three different laboratories, whereas during the other periods strains were isolated in one centre. Metronidazole, chloramphenicol, imipenem and amoxicillin/clavulanic acid were the most active drugs. A high level of resistance was observed in 1987 for ampicillin (88% at > 4 mg/L), erythromycin (51% at > 4 mg/L), tetracyclin (53% at > 8 mg/L) and clindamycin (27% at > 4 mg/L). The same level of resistance was seen during the further years for clindamycin and ampicillin. Resistance to cefoxitin increased from 6% to 11% between 1987 and 1993/1994. No differences in resistance rates were observed between the strains collected in the three centers. For 100 strains, the results of the E test were compared with those of the micro-broth dilution test, both being used routinely for testing the antibiotic susceptibility of Bacteroides fragilis group strains in this period.     

Effects of subinhibitory concentrations of clindamycin on the morphological, biochemical and genetic characteristics of Bacteroides fragilis

The effects of subinhibitory concentrations of clindamycin on the morphological, biochemical and genetic characteristics of species of the Bacteroides fragilis group isolated from children with diarrhea were determined. The minimal inhibitory and subinhibitory concentrations for clindamycin were determined. Minimal inhibitory concentration values ranging from 0.25 to 512 microg mL(-1) were observed. Cultures grown with clindamycin were used to determine the macroscopic morphological characteristics, cellular viability, ultrastructural characteristics and DNA integrity. Clindamycin did not alter colonial morphology, but after 6 h elongated cells were observed. Also, extracellular vesicles and electron-lucent areas inside the cytoplasm were observed. Bacteria treated with clindamycin also showed fragmentation of DNA as determined by electrophoresis. The alterations produced by clindamycin might be indicative of a possible modification of the structures involved in bacterial pathogenesis.     

Antibiotic Susceptibility of Bacteroides fragilis Group Strains in Hungary

Resistance rates to different antibiotics of 495 Bacteroides fragilis group strains were followed between 1987 and 1994 in Hungary. In 1992 the strains were collected in three different laboratories, whereas during the other periods strains were isolated in one centre. Metronidazole, chloramphenicol, imipenem and amoxicillin/clavulanic acid were the most active drugs. A high level of resistance was observed in 1987 for ampicillin (88% at > 4 mg/L), erythromycin (51% at > 4 mg/L), tetracyclin (53% at > 8 mg/L) and clindamycin (27% at > 4 mg/L). The same level of resistance was seen during the further years for clindamycin and ampicillin. Resistance to cefoxitin increased from 6% to 11% between 1987 and 1993/1994. No differences in resistance rates were observed between the strains collected in the three centers. For 100 strains, the results of the E test were compared with those of the micro-broth dilution test, both being used routinely for testing the antibiotic susceptibility of Bacteroides fragilis group strains in this period.     

Restored fitness leads to long-term persistence of resistant Bacteroides strains in the human intestine

Acquired antibiotic resistance typically confers a cost to the bacteria, but these costs can be reduced by genetic compensation over time. The fitness of two Bacteroides thetaiotaomicron clones consecutively isolated in vivo was studied using an in vitro pair-wise competition method. The isolates derived from faecal samples of two clindamycin-exposed healthy volunteers and the two B. thetaiotaomicron clone types could be followed up to 18 months in these two subjects. The two clones were originally susceptible to clindamycin and lacked erm genes; however, after 7 days of clindamycin administration they carried the erm (erythromycin methylase)(G) or (F) gene, respectively, and expressed phenotypic clindamycin resistance. The initial cost of acquired resistance was high as seen in the in vitro pair-wise competition experiments. At 2 weeks post-administration, no growth disadvantage was detected for isolates of either of the two clones in the in vitro experiments and this regained fitness remained for isolates collected up to 18 months. Competition analysis of an in vitro isolated erm(G) positive transconjugant also demonstrated an initial reduction of fitness that was restored over time. The results indicate that the biological cost associated with a resistance gene can rapidly be compensated during in vivo growth. Thus, once the resistant clone has gained its resistance determinant it will be difficult to eliminate.     

Description of Complex Forms of a Porin in Bacteroides fragilis and Possible Implication of this Protein in Antibiotic Resistance

Periodic surveys of antibiotic susceptibility patterns among anaerobes have emphasized that new mechanisms of resistance have emerged, especially in the Bacteroides fragilis group. Resistance to the combination of amoxicillin and clavulanic acid among some imipenem-susceptible Bacteroides fragilis strains has been associated with modifications in outer membrane protein electrophoretic patterns with the loss of some porin-like proteins. Porins are outer membrane proteins that play a major part in membrane permeability; if they are under-expressed, they can be responsible for antibiotic resistance. In a previous work, we isolated one outer membrane protein of 45 kDa from Bacteroides fragilis and showed its porin activity. In the present study, we aim to isolate the different complex forms of this protein and to underline their possible role in antibiotic resistance. We therefore compared the electrophoretic patterns of the outer membrane proteins of several strains of Bacteroides fragilis. Although these patterns are similar to each other, some proteins, especially those of high molecular weight, are less visible in the samples heated before electrophoresis. We targeted these high molecular weight proteins (which appeared sensitive to heat) and isolated them by electro-elution. We thus identified two high molecular weight proteins (210 and 130/135 kDa) which seemed to be components of a complex including the 45 kDa outer membrane protein formerly identified by us as a porin protein. Their porin activities were tested by the swelling assay of proteoliposomes which showed that the 210 kDa protein behaved like the 45 kDa protein whereas the 130/135 kDa protein had less porin activity. Furthermore, swelling assays with antibiotic solutions made it possible to compute the role of this protein complex in antibiotic resistance.     

Evidence for extensive resistance gene transfer among Bacteroides spp. and among Bacteroides and other genera in the human colon

Transfer of antibiotic resistance genes by conjugation is thought to play an important role in the spread of resistance. Yet virtually no information is available about the extent to which such horizontal transfers occur in natural settings. In this paper, we show that conjugal gene transfer has made a major contribution to increased antibiotic resistance in Bacteroides species, a numerically predominant group of human colonic bacteria. Over the past 3 decades, carriage of the tetracycline resistance gene, tetQ, has increased from about 30% to more than 80% of strains. Alleles of tetQ in different Bacteroides species, with one exception, were 96 to 100% identical at the DNA sequence level, as expected if horizontal gene transfer was responsible for their spread. Southern blot analyses showed further that transfer of tetQ was mediated by a conjugative transposon (CTn) of the CTnDOT type. Carriage of two erythromycin resistance genes, ermF and ermG, rose from <2 to 23% and accounted for about 70% of the total erythromycin resistances observed. Carriage of tetQ and the erm genes was the same in isolates taken from healthy people with no recent history of antibiotic use as in isolates obtained from patients with Bacteroides infections. This finding indicates that resistance transfer is occurring in the community and not just in clinical environments. The high percentage of strains that are carrying these resistance genes in people who are not taking antibiotics is consistent with the hypothesis that once acquired, these resistance genes are stably maintained in the absence of antibiotic selection. Six recently isolated strains carried ermB genes. Two were identical to erm(B)-P from Clostridium perfringens, and the other four had only one to three mismatches. The nine strains with ermG genes had DNA sequences that were more than 99% identical to the ermG of Bacillus sphaericus. Evidently, there is a genetic conduit open between gram-positive bacteria, including bacteria that only pass through the human colon, and the gram-negative Bacteroides species. Our results support the hypothesis that extensive gene transfer occurs among bacteria in the human colon, both within the genus Bacteroides and among Bacteroides species and gram-positive bacteria.