Postantibiotic effects with Bacteroides fragilis determined by viable counts and CO2 generation

Abstract

OBJECTIVE: To study the postantibiotic effect (PAE) for Bacteroides fragilis after exposure to common anaerobic antimicrobials with two different methods, by viable counting and by measuring CO2 generation in a BACTEC(R) blood culture system. METHODS: Four strains of B. fragilis were exposed for 1, 2 and 4 h to cefoxitin, chloramphenicol, clindamycin, imipenem or metronidazole at concentrations from 1 to 16 x MIC. The drugs were removed by dilution into BACTEC 7A(R) vials and growth determined with viability counts and CO2 production. RESULTS: The durations of the PAEs determined by the two methods correlated well (r=0.913, p<0.005). PAEs of up to 4-5 h were induced by imipenem and metronidazole with achievable concentrations and exposure durations. Chloramphenicol induced short or no PAEs, but cefoxitin and clindamycin induced PAEs up to 2 h with high AUC values. The imipenem PAEs and the short cefoxitin and clindamycin PAEs were dependent on AUC. CONCLUSIONS: Significant PAEs against B. fragilis were induced by imipenem and metronidazole. Determining PAE by measuring CO2 production is an accurate and less time-consuming alternative to the conventional method of viable counts.

Postantibiotic effect in vitro

“…Moreover, a definite result could be obtained after only five days as compared to 10-14 days with viable counting (I). Similar high concordances on duration of PAE between C02 generation in a nonradioactive culture system and viable counting (r>O.90) have been reported with strains of Staphylococcus aureus and Escherichia coli (Gottfredsson et al 1991a), Helicobacterpylorii (Craig & Gudmundsson, 1996) or Bacteroides fragilis (Valdimarsdottir et al 1997). The growth index detected by the instrument is a combination of the lag before regrowth and the bacterial inoculum which signifies that the inoculum in both the drug-treated and in the control culture should be the same.…”

“…Data on other pathogens are less well studied. Inhibition of regrowth has been observed on anaerobics where metronidazole induces the most appreciable PAEs, clindamycin exhibits intermediate PAEs, while betalactams produce variable results (Valdimarsdottir et al 1997;Yagi et al 1992;Garcia-Rodriguez et al 1991;Neu et al 1987) Data on yeast have shown that amphotericin B induces consistent PAEs while no PAEs are induced with the azoles (Craig & Gudmundsson, 1996;Scheven et al 1995;Turnidge et al 1994).…”

Post-pH effect in oral streptococci

“…The oral streptococci able to survive at a low pH and to continue producing acid under these conditions are thought to be the most cariogenic[4]. With some exceptions, the ability to grow at pH 5.5 is considered to mark the difference between mutans and non‐ mutans acidophilic streptococci[5–21]. The rate of acid production and the aciduric capacity also vary among species[22]; both properties are thought to be strain dependent[7].…”

“…The objective of our study was to determine the capacity of oral streptococci to resume growth after undergoing pH shock. This parameter, which we call the 'post-pH effect' (PpHE), was defined along the lines established by other authors for determining postantibiotic effect (with the Bactec system) [16] and post-H 2 O 2 effect (with the BacT/Alert system) [17]. The PpHE is proposed as an in vitro ecological determinant to help assess the cariogenic potential of oral microflora.…”

Pharmacodynamic Studies of Vancomycin, Metronidazole and Fusidic Acid against <i>Clostridium difficile</i>

“…Metronidazole showed PAEs between 1.2 and 1.7 h with long PA SMEs against two of the three strains. In a study by Valdimarsdottir et al [33] , a PAE of 4-5 h was noted for metronidazole against Bacteroides fragilis . The concentrations in the killing experiments can be compared with what is obtained in humans.…”