This image is an x1,000 magnification of Lactobacillus sp. adhering to a vaginal epithelial cell as seen on a Gram stain of vaginal fluid. The sample was collected by rolling a swab along the vaginal wall and then onto a slide. Lactobacilli are facultatively anaerobic gram-positive rods which make up 90% of the normal human vaginal microflora. They are present in quantities of 10⁷ to 10⁸ CFU/g of vaginal fluid and serve as regulators of the vaginal ecosystem. Their presence has been demonstrated to decrease the incidence of bacterial vaginosis, gonorrhea, urinary tract infections, and HIV. Adherence of Lactobacillus sp. to vaginal epithelial cells, as depicted here, may prevent urogenital infections by competitively excluding attachment and vaginal colonization by other microorganisms.   Other mechanisms by which Lactobacillus sp. contribute to the vaginal host defense system include production of lactic acid, bacteriocins, and hydrogen peroxide.