Antibiotic susceptibility of potentially probiotic Lactobacillus species

In recent years, the time-honored reputation of lactobacilli as promoters o f gastrointestinal and female urogenital health has been qualified. This ha s occurred due to a rare association with human infection in the presence o f certain predisposing factors and their potential to act as a source of un desirable antibiotic resistance determinants to other members of the indige nous microbiota. This necessitates greater caution in their selection for u se in microbial adjunct nutrition and disease management (prophylaxis and t herapy). It was against this background that 46 Lactobacillus strains from human and dairy sources were assayed for susceptibility to 44 antibiotics. All strains were resistant to a group of 14 antibiotics, which included inh ibitors of cell wall synthesis (cefoxitin [30 mu g] and aztreonam [30 mu g] ), protein synthesis (amikacin [30 mu g], gentamicin [10 mu g], kanamycin [ 30 mu g], and streptomycin [10 mu g]), nucleic acid synthesis (norfloxacin [10 mu g], nalidixic acid [30 mu g], sulphamethoxazole [100 mu g], trimetho prim [5 mu g], co-trimoxazole [25 mu g], and metronidazole [5 mu g]), and c ytoplasmic membrane function (polymyxin B [300 mu g] and colistin sulphate [10 mu g]). All strains were susceptible to tetracycline (30 mu g), chloram phenicol (30 mu g), and rifampicin (5 mu g) Four human strains and one dair y strain exhibited atypical resistance to a penicillin, bacitracin (10 mu g ), and/or nitrofurantoin (300 mu g) One human strain was also resistant to erythromycin (15 mu g) and clindamycin (2 mu g). These resistances may have been acquired due to antibiotic exposure in vivo, but conclusive evidence is lacking in this regard. Seven microorganism-drug combinations were evalu ated for beta-lactamase activity using synergy and nitrocefin tests. The ab sence of activity suggested that cell wall impermeability appeared responsi ble for beta-lactam resistance. The occurrence of a minority of lactobacill i with undesirable, atypical resistance to certain antibiotics demonstrates that not all strains are suitable for use as probiotics or bacteriotherape utic agents. The natural resistance of lactobacilli to a wide range of clin ically important antibiotics may enable the development of antibiotic/probi otic combination therapies for such conditions as diarrhea, female urogenit al tract infection, and infective endocarditis.