Lactobacillus

Lactobacillus spp. are gram-positive rods, catalase-negative, and typically do not grow well on routine blood agar aerobically (microaerophilic/anaerobic preference), distinguishing them from other gram-positive rods like Corynebacterium or Listeria. In the vaginal microbiome, Lactobacillus (especially L. crispatus, L. jensenii) ferments glycogen from vaginal epithelial cells into lactic acid, maintaining an acidic environment (pH <4.5) that inhibits growth of Gardnerella vaginalis, Candida, and other pathogens—this is clinically relevant in bacterial vaginosis, where Lactobacillus is replaced by Gardnerella and anaerobes, causing a rise in vaginal pH (>4.5) and a 'fishy' amine odor with KOH (whiff test). Lactobacillus is a key diagnostic clue on wet mount—its presence with normal vaginal flora argues against BV, while its paucity with clue cells supports BV diagnosis. It is part of normal GI flora and contributes to fermentation (yogurt, probiotics, sauerkraut). Clinically, Lactobacillus is essentially nonpathogenic but rare cases of bacteremia/endocarditis have been reported in immunocompromised patients, especially with central lines. It is intrinsically resistant to vancomycin (unlike Enterococcus), which can be a useful distinguishing lab feature in differentiating catalase-negative, gram-positive organisms. Lactobacillus GG and other probiotic strains have been studied for preventing antibiotic-associated diarrhea and in management of irritable bowel syndrome. Understanding Lactobacillus's protective role in vaginal health is tested frequently in the context of differentiating normal flora from BV, trichomoniasis, and candidiasis on Step 1/Step 2.