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Rouquette C. Berche P. The pathogenesis of infection by Listeria monocytogenes Microbiologia. 12:245-58, 1996 Listeria monocytogenes is a Gram-positive bacterium responsible for severe infections in human and a large variety of animal species. It is a facultative intracellular pathogen which invades macrophages and most tissue cells of infected hosts where it can proliferate. The molecular basis of this intracellular parasitism has been to a large extent elucidated. The virulence factors, including internalin, listeriolysin O, phospholipases and a bacterial surface protein, ActA, are encoded by chromosomal genes organized in operons. Following internalisation into host cells, the bacteria escape from the phagosomal compartment and enter the cytoplasm. They then spread from cell to cell by a process involving actin polymerisation. In infected hosts, the bacteria cross the intestinal wall at Peyer's patches to invade the mesenteric lymph nodes and the blood. The main target organ is the liver, where the bacteria multiply inside hepatocytes. Early recruitment of polymorphonuclear cells lead to hepatocyte lysis, and thereby bacterial release This causes prolonged septicaemia, particularly in immunocompromised hosts, thus exposing the placenta and brain to infection. The prognosis of listeriosis depends on the severity of meningoencephalitis, due to the elective location of foci of infection in the brain stem (rhombencephalitis). Despite bactericidal antibiotic therapy, the overall mortality is still high (25 to 30%).

Identification of bacteria in direct smears may be of lifesaving importance. For example, a rapid diagnosis of bacterial meningitis, made after examining a gram stained smear of the patient's cerebrospinal fluid, allows the physician to begin treatment immediately.The appearance of bacteria on gram stained smears is suggestive of a certain species, but identification may not be made on the basis of the stain alone. An exception to this rule is the presence of gram negative intracellular diplococci from a male urogenital specimen, which is presumptive identification of Niesseria gonorrhoeae.In addition, culture results can be correlated with the direct smear report.

Direct smears read specifically for the presence of gonococci should include a direct reference to gram negative intracellular diplococci.

Gram negative cocci which occur in pairs with their adjacent sides flattened, giving them a coffee bean appearance, are typical of the genus Neisseria. Neisseria gonorrhea is commonly found within white blood cells; these organisms must be carefully sought within genital tract specimens, and their presence or absence should be specifically noted in the report.The presence of intracellular gram negative bacilli in a purulent male urethral smear is presumptive evidence of gonococci, but this assumption cannot be made in a female. Beware of other organisms which resemble gonococci but are found extracellularly, such as Acinetobacter lwoffi, which must be distinguished biochemically.