Ampicillin and ampicillin-sulbactam dilution tests with mixed cultures ofBacteroides fragilis, Escherichia coli andEnterococcus

Abstract

Bacterial interactions in mixed infections may compromise antimicrobial therapy. The in vitro bactericidal activity of ampicillin and ampicillin-sulbactam against aerobic/anaerobic mixed cultures of Bacteroides fragilis, Escherichia coli and Enterococcus spp. was studied by means of broth dilution tests. The MBC of ampicillin for Enterococcus faecalis 6 was 0.25 mg/l when tested singly; in association with B. fragilis 1, however, the MBC for E. faecalis 6 was 16 mg/l. When tested singly E. coli 9 and B. fragilis 1 were both killed by ampicillin at a concentration of 4 mg/l in combination with 1 mg/l sulbactam. When both strains were associated, the MBC for B. fragilis 1 rose to > 256 mg/l. Results obtained indicate that the behaviour of bacteria as determined in pure cultures is not necessarily identical with the antibiotic susceptibility present in mixed cultures. It was shown that, besides beta-lactamase production by involved bacteria, other factors contribute to the alteration of bacterial susceptibility in mixed cultures. Synergistic and antagonistic effects between associated organisms were observed. For example, it was found that two moderately sensitive bacterial strains were resistant in mixed culture.

In Vitro Activity of Trovafloxacin against Bacteroides fragilis in Mixed Culture with either Escherichia coli or a Vancomycin- Resistant Strain of Enterococcus faecium Determined by an Anaerobic Time-Kill Technique

“…However, the inactivation of trovafloxacin by VREF or E. faecium was not found in the present study since approximately 94% of trovafloxacin activity was recovered from culture supernatants after a 24-h incubation compared to the activity of a bacteria-free control. Another possible explanation could be the production of an antagonistic substance by the enterococcal strains (5,15) that could interfere with the activity of trovafloxacin. We have found, however, that culture supernatants from VREF and E. faecium failed to confer protection on B. fragilis ATCC 23745 (results not shown).…”

“…However, the killing of bacteria in pure culture may not necessarily be identical to the killing in mixed culture (15,22). We have developed a mixed culture anaerobic time-kill technique for a comparative study of the in vitro activity of trovafloxacin against B. fragilis in pure or mixed culture with either E. coli or a vancomycin-resistant strain of Enterococcus faecium (VREF).…”

A microfluidic approach to study the effect of bacterial interactions on antimicrobial susceptibility in polymicrobial cultures

“…25 Because the bacterial interactions can inuence susceptibility, a few of these studies have focused on the effects of bacterial interaction on antimicrobial susceptibility in polymicrobial cultures. 26,27 In these studies, changes in antimicrobial susceptibility of polymicrobial cultures (B. fragilis, E. coli and E. faecalis) have been primarily attributed to the production of enzymes such as blactamases, which protect co-existing bacteria. A few studies also have focused on manipulation of inter-species interaction to inuence susceptibility.…”

Antibiotic Susceptibility of Cocultures in Polymicrobial Infections Such as Peri‐Implantitis or Periodontitis: An In Vitro Model

“…An in vitro study 15 showed the pathogen Bacteroides fragilis as a pure culture could be treated better by the antibiotic trovafloxacin than in a coculture in the presence of the pathogen Enterococcus faecium . Heilmann 19 found similar results in an in vitro study whereby the coculture out of the two pathogens B. fragilis and Escherichia coli showed a complete resistance to the antibiotic AM with sulbactam, although the isolates were sensitive to this antibiotic. However, the majority of studies about mixed infections of the respiratory, 4,20‐22 gastrointestinal, 5 ,15‐17, 23 and urogenital tract, 14 and a study on peri‐implant mixed infections, 18 demonstrated that interactions among pathogens led, in most cases, to the increased antibiotic resistance of the coculture compared to pure cultures.…”