Susceptibility of 40 Lactobacilli to Six Antimicrobial Agents with Broad Gram-Positive Anaerobic Spectra

Abstract

The minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of 40 lactobacillus strains were determined against six antibiotics with broad anaerobic spectra. Penicillin, ampicillin, clindamycin, and cephalothin were the most active inhibitory agents, with 95 to 100% of the strains inhibited at clinically achievable serum levels. However, despite the inhibitory efficacy of these four agents, only 5 to 22% of the isolates were killed at achievable concentrations. MBC:MIC ratios were high, ranging from 30:1 for cephalothin to 266:1 for ampicillin. Cefoxitin and metronidazole were generally ineffective against lactobacilli, with 87.5 to 100% of strains having unachievable MICs and/or MBCs. These findings may partially explain the clinical observations noting the inability to eradicate endocarditic lactobacillemias despite readily achievable MICs.Lactobacilli are being increasingly recognized as important pathogens in human infections (1,3,17,20). An intriguing phenomenon noted in many previously cited cases of lactobacillemia was the therapeutic difficulty in eradicating this organism from deep-seated infected foci, especially the endocardium, despite seemingly appropriate treatment regimens and serum antibiotic concentrations exceeding the minimal inhibitory concentrations (MICs).To investigate this problem of apparent discrepancy between in vitro susceptibility of lactobacilli and poor in vivo therapeutic responses, the MICs and minimal bactericidal concentrations (MBCs) of six antibiotics with broad grampositive anaerobic spectra were determined by a modified broth dilution technique for 40 clinical isolates. MATERLALS AND METHODSIsolation of the lactobacilli. All 40 strains of lactobacilli were clinical isolates from a variety of body sites, including blood, genital tract, oropharynx, ascitic fluid, lymph node, and brain tissue. Ten were obtained from the clinical laboratory of Harbor General Hospital, whereas the remaining 30 organisms were kindly provided by the following investigators: Elizabeth P. Cato, Blacksburg, Va.; Robert E. Weaver, Atlanta, Ga.; Jack P. London, Bethesda, Md.; and M. Elisabeth Sharpe, Reading, England.The lactobacilli were identified by typical appearance on Gram stain and by biochemical reactions according to Bergey's criteria (14). Identification of species was according to classifications of Holdeman and Moore (12). The following strains of lactobacilli were used in this study: Lactobacillus casei (24 strains), L. plantarum (5 strains), L. acidophilus and L. minutis (3 strains each), L. fermentans and L. brevis (2 strains each), and L. leichmanii (1 strain). All of the L. plantarum and L. casei and two of the three L. minutis isolates were facultative anaerobes; the remainder were strict anaerobes.Organisms were maintained in chopped-meat glucose (CMG) broth (Scott Laboratories, Fiskeville, R.I.) and were transferred monthly into fresh media until the susceptibility testing was performed. In the week before testing, one additional transfer was done.Pre...

Effect of Metronidazole on the Growth of Vaginal Lactobacilli in vitro

“…However, the authors recovered lactobacilli from only 65% of the women 1 month after treatment. In the study performed by Bayer and colleagues 12 , metronidazole was totally ineffective against the lactobacilli. However, the maximal concentration they tested was 320 mg/ml, based on the generally achievable serum concentration of 12.5 mg/ml after the administration of oral metronidazole.…”

The Inhibitory Effect of Clindamycin on Lactobacillus in vitro

“…But growth and dominance of Lactobacillus was restored in a month 12,13 . The in vitro study conducted by Bayer and colleagues 14 determined that 98% of Lactobacillus strains were inhibited by clindamycin. The present study was performed to evaluate the in vitro effect of varying concentrations of clindamycin on Lactobacillus spp.…”

Evaluation of New Broth Media for Microdilution Antibiotic Susceptibility Testing of Lactobacilli, Pediococci, Lactococci, and Bifidobacteria

“…A mixed formulation of Iso-Sensitest broth (90%) and deMan-Rogosa-Sharpe broth (10%) with or without supplementation with L-cysteine, referred to as the LAB susceptibility test medium, provided the most optimal medium basis in terms of growth support of nonenterococcal LAB and correct indication of MICs of international control strains.A large variety of methods to determine antibiotic susceptibilities of nonenterococcal lactic acid bacteria (LAB) belonging to the genera Lactobacillus, Pediococcus, Lactococcus, and Bifidobacterium based on either agar disk diffusion (4, 5, 15, 20, 26, 29, 32, 33, 35), E-test (6, 9, 10, 13, 15, 16, 19, 21), agar dilution (3,7,11,17,19,22) or broth dilution (1,12,14,18,23,24,25,27,29,30,31, 34) has been described. Due to the fact that many of these organisms require special growth conditions in terms of medium acidity and nutrient supplementation, conventional media such as Mueller-Hinton and Iso-Sensitest (IST) agar or broth are not uniformly suitable for susceptibility testing of nonenterococcal LAB.…”

“…A large variety of methods to determine antibiotic susceptibilities of nonenterococcal lactic acid bacteria (LAB) belonging to the genera Lactobacillus, Pediococcus, Lactococcus, and Bifidobacterium based on either agar disk diffusion (4,5,15,20,26,29,32,33,35), E-test (6,9,10,13,15,16,19,21), agar dilution (3,7,11,17,19,22) or broth dilution (1,12,14,18,23,24,25,27,29,30,31,34) has been described. Due to the fact that many of these organisms require special growth conditions in terms of medium acidity and nutrient supplementation, conventional media such as Mueller-Hinton and Iso-Sensitest (IST) agar or broth are not uniformly suitable for susceptibility testing of nonenterococcal LAB.…”