Listeriosis usually does not present clinically until there is disseminated infection. In food- borne outbreaks, sometimes gastrointestinal manifestations of primary infection such as nausea, abdominal pain, diarrhea, and fever occur. Disseminated infection in adults is usu- ally occult, involving fever, malaise, and constitutional symptoms without an obvious focus. L. monocytogenes has a tropism for the central nervous system (CNS), including the brain parenchyma (encephalitis) and brainstem, but the meningitis it causes is not distinct from that associated with other leading bacterial pathogens (Streptococcus pneumoniae, Neisseria meningitidis).
Neonatal and puerperal infections appear in settings similar to those of infections with group B streptococci. Intrauterine infection leads to stillbirth or a disseminated infection at or near birth. If the pathogen is acquired in the birth canal, the onset of disease is later. The risk of disease is increased in elderly and immunocompromised individuals as well as women in late pregnancy. The number of cases in AIDS patients has been estimated at 300 times the general population.
Diagnosis of listeriosis is by culture of blood, cerebrospinal fluid (CSF), or focal lesions. In meningitis, CSF Gram stains are usually positive. The first indication thatListeria is involved is often the discovery that the β-hemolytic colonies subcultured from a blood culture bottle are Gram-positive rods rather than streptococci.
L. monocytogenes is susceptible to penicillin G, ampicillin, and trimethoprim/sul- famethoxazole, all of which have been used effectively. Ampicillin combined with gen- tamicin is considered the treatment of choice for fulminant cases. Intense surveillance to prevent the sale of Listeria-contaminated ready-to-eat meat products has led to a marked decrease in the incidence of new infections. There is no vaccine.
The genus Bacillus includes many species of aerobic or facultative, spore-forming, Gram- positive rods. With the exception of one species, B. anthracis, they are low-virulence saprophytes widespread in air, soil, water, dust, and animal products. B. anthracis causes the zoonosis anthrax, a disease of animals that is occasionally transmitted to humans.
The genus is made up of rod-shaped organisms that can vary from coccobacillary to rather long-chained filaments. Motile strains have peritrichous flagella. Formation of round or oval spores, which may be central, subterminal, or terminal depending on the species, is characteristic of the genus. Bacillus species are Gram positive; however, posi- tivity is often lost, depending on the species and the age of the culture.
With Bacillus, growth is obtained with ordinary media incubated in air and is reduced or absent under anaerobic conditions. The bacteria are catalase positive and metabolically active. The spores survive boiling for varying periods and are sufficiently resistant to heat that those of one species are used as a biologic indicator of autoclave efficiency. Spores of B. anthracis survive in soil for decades.