Sonja Löfmark

Long-term ecological impacts of antibiotic administration on the human intestinal microbiota

Antibiotic administration is known to cause short-term disturbances in the microbiota of the human gastrointestinal tract, but the potential long-term consequences have not been well studied. The aims of this study were to analyse the long-term impact of a 7-day clindamycin treatment on the faecal microbiota and to simultaneously monitor the ecological stability of the microbiota in a control group as a baseline for reference. Faecal samples from four clindamycin-exposed and four control subjects were collected at nine different time points over 2 years. Using a polyphasic approach, we observed highly significant disturbances in the bacterial community that persisted throughout the sampling period. In particular, a sharp decline in the clonal diversity of Bacteroides isolates, as assessed by repetitive sequence-based PCR (rep-PCR) and long-term persistence of highly resistant clones were found as a direct response to the antibiotic exposure. The Bacteroides community never returned to its original composition during the study period as assessed using the molecular fingerprinting technique, terminal restriction fragment length polymorphism (T-RFLP). Furthermore, using real-time PCR we found a dramatic and persistent increase in levels of specific resistance genes in DNA extracted from the faeces after clindamycin administration. The temporal variations in the microbiota of the control group were minor compared to the large and persistent shift seen in the exposed group. These results demonstrate that long after the selection pressure from a short antibiotic exposure has been removed, there are still persistent long term impacts on the human intestinal microbiota that remain for up to 2 years post-treatment.

Metronidazole Is Still the Drug of Choice for Treatment of Anaerobic Infections

Metronidazole has been used for the treatment of infections for >45 years and is still successfully used for the treatment of trichomoniasis, amoebiasis, and giardiasis. Anaerobic bacterial infections caused by Bacteroides species, fusobacteria, and clostridia respond favorably to metronidazole therapy. Good clinical results in the treatment of vaginosis due to Gardnerella vaginalis have also been reported. Rates of resistance to metronidazole are still generally low; however, several studies have reported decreased susceptibility among Bacteroides species, as well as different mechanisms of resistance. Metronidazole-resistant Helicobacter pylori strains have been described, but combination therapy (eg, metronidazole, amoxicillin, or clarithromycin plus omeprazole) is still recommended for eradication of this pathogen in patients with gastroduodenal ulcers. Metronidazole is considered to be a cost-effective drug because of its low cost, good activity against pathogenic anaerobic bacteria, favorable pharmacokinetic and pharmacodynamic properties, and minor adverse effects. Metronidazole is still the criterion standard for therapy of anaerobic infections, as was described by Tally and colleagues 35 years ago.

Inducible Metronidazole Resistance and nim Genes in Clinical Bacteroides fragilis Group Isolates

ABSTRACT Nitroimidazole resistance (nim) genes were detected in 2% of 1,502 clinical Bacteroidesfragilis group strains isolated from 19 European countries, and a novel nim gene was identified. High metronidazole resistance could be induced in nim-positive strains, which emphasizes the importance of acknowledging metronidazole resistance in the clinical setting.

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.

Long-term antimicrobial resistance in Escherichia coli from human intestinal microbiota after administration of clindamycin

The aim of this study was to gain better knowledge of how the intestinal microbiota are affected over time after administration of an antimicrobial agent. This study monitored the prevalence and frequencies of antibiotic resistance in Enterobacteriaceae against 17 antimicrobial agents, during a 2-y period, in consecutive faecal samples collected from 8 healthy volunteers. Four subjects had received 150 mg clindamycin perorally for 7 d, while 4 non-exposed subjects served as a control group. The samples from both groups were cultured and screened for Enterobacteriaceae. The highest incidence of resistance observed was to ampicillin. The ampicillin resistance is due to production of the beta-lactamase TEM-1. The administration of clindamycin had a prolonged impact on the composition of the microbiota, even though enterobacteria are intrinsically resistant to clindamycin; the level of resistance in Escherichia coli isolates was elevated after administration and persisted up to 9 months after administration. After 9 months the susceptibility levels in the exposed group were similar to those at d 0.

Prolonged impact of a one-week course of clindamycin on Enterococcus spp. in human normal microbiota.

Human intestinal Enterococcus spp. was monitored during a 2-y period after 7 d clindamycin treatment. Consecutive faecal samples were collected from 8 healthy volunteers, 4 of whom had received clindamycin. After treatment, the number of enterococcal colonies was diminished and species variation extended. Erythromycin and clindamycin resistance increased from 19% to 69% and 0% to 67%, respectively. Elevated resistance levels lasted up to 9 months and erm(B) was detected in samples up to 6 months. Our results show that the clindamycin treatment had a prolonged impact on resistance and species variation.