The laboratory investigation of vaginal discharge.

The female genital tract has a complex microbial flora which varies with site, age of the patient, and hormonal background. Likewise, the quality and quantity of cervical and vaginal secretions vary with many factors including age, phase of the menstrual cycle, use of oral contraceptive, pregnancy, and presence of an intrauterine contraceptive device. Vaginal discharge may arise from the vagina, cervix, or upper genital tract. There are many noninfective causes of vaginal discharge; however, it is the role of the diagnostic microbiology laboratory to determine the presence of recognised microbial pathogens and disturbances of the normal flora. The normal flora of the vagina changes according to circulating oestrogen levels. When oestrogen is present, the vaginal epithelium contains glycogen which favours colonisation by lactobacilli and other acid tolerant bacteria, which metabolise glycogen to lactic acid. This results in a pH of 4.5 or less. Other bacteria commonly present in large numbers include anaerobic streptococci, diphtheroids, coagulase negative staphylococci, and á haemolytic streptococci. Other common commensals which can act as pathogens include Candida spp, Staphylococcus aureus, â haemolytic streptococci including Str agalactiae, and Actinomyces spp. In bacterial vaginosis the concentration of Gardnerella vaginalis, Mobiluncus spp, anaerobic gram negative rods (for example, Bacteroides spp, Prevotella spp, and Porphyromonas spp) and Mycoplasma hominis increases, compared with the lactobacilli. The vaginal secretions of prepubertal children and postmenopausal women are more alkaline and contain a varied bacterial flora dominated by coagulase negative staphylococci, diphtheroids, á haemolytic streptococci, lactobacilli, non-haemolytic streptococci, anaerobic Gram negative rods, and Escherichia coli.