|please click the image for large view in new window
Listeria are important Gram-positive organisms that can grow at
low temperatures (4-10 �C); Listeria monocytogenes is associated
with human disease.
Listeria spp. are found in soil or animal faeces and can contaminate
foodstuffs. Infection follows consumption of contaminated
food; inadequately pasteurized foods and contaminated foods
stored in the fridge are at risk.
Listeria monocytogenes infection is usually a mild, self-limiting,
infectious mononucleosis-like syndrome. Rarely acute pyogenic
meningitis, bacteraemia or encephalitis can develop and these conditions
carry a high mortality rate, particularly in patients with
reduced cell-mediated immunity. Bacteraemia occurring in pregnancy
is associated with intrauterine death, premature labour and
neonatal infection similar to that seen with group B streptococci
(see Congenital and perinatal infections
Diagnosis is by culture on simple laboratory media and further
identification is made by biochemical testing. Typing is achieved
by multilocus sequence typing (MLST).
Listeria spp. are susceptible to ampicillin and gentamicin but
resistant to the cephalosporins, penicillin and chloramphenicol.
Patients with symptoms of meningitis, in whom listeriosis is a possible
diagnosis, should receive a drug regimen that incorporates
Prevention and control
Listeriosis is controlled by food hygiene, effective refrigeration and
adequate reheating of pre-prepared food. Individuals at particular
risk, such as pregnant women and immunocompromised patients,
should avoid high-risk foods.
This organism, which is naturally resistant to most antibiotics
except vancomycin, colonises prostheses and intravenous lines
causing infection and bacteraemia, usually in immunocompromised
Other corynebacteria and related organisms
Rarely, Corynebacterium ulcerans can carry the phage that encodes
diphtheria toxin and may cause a diphtheria-like pharyngitis.
Corynebacterium pseudotuberculosis may cause suppurative granulomatous
lymphadenitis. Rhodococcus equi has been associated
with a severe cavitating pneumonia in patients with acquired
immune deficiency syndrome (AIDS).
Different species may cause localized or disseminated disease
in immunocompromised patients. Some may infect prosthetic
Mycobacterium avium-intracellulare complex
The mycobacterium avium-intracellulare complex (MAIC)
includes Mycobacterium avium, M. intracellulare and M. scrofulaceum,
some being natural pathogens of birds, others being environmental
saprophytes. They are a common cause of mycobacterial
lymphadenitis in children, also causing osteomyelitis in immunocompromised
patients and chronic pulmonary infection in the
elderly. In the advanced stages of AIDS, they cause disseminated
infection and bacteraemia. MAIC is naturally resistant to many
antituberculosis agents and treatment with multidrug regimens
that include rifabutin, clarithromycin and ethambutol is usually
required. Lymphadenitis may require surgery.
Mycobacterium kansasi, Mycobacterium malmoense and Mycobacterium xenopi
These species cause an indolent pulmonary infection that resembles
tuberculosis in individuals predisposed by chronic lung disease
that has caused deranged pulmonary anatomy (e.g. bronchiectasis,
silicosis and obstructive airways disease). Initial therapy with
standard drugs may have to be adjusted following the results of
bacterial identification and susceptibility tests.
The spores produced by these Gram-positive aerobic bacilli allow
them to survive in adverse environmental conditions.
Bacillus anthracis is a soil organism that, under certain climatic
conditions, multiplies to cause anthrax in herbivores. Humans can
become infected from contaminated animal products. Pathogenicity
depends on three bacterial antigens: the 'protective antigen',
the oedema factor (both of which are toxins) and the antiphagocytic
poly D-glutamic acid capsule. Inoculation of B. anthracis into
minor skin abrasions produces a necrotic, oedematous ulcer with
regional lymphadenopathy. Inhalation of anthrax spores develops
into fulminant pneumonia and septicaemia. An outbreak in 2001
that was caused by deliberate release of the organism has led to
the recognition of anthrax as an agent of bioterrorism. The spores
of the organisms are prepared in a way that makes them readily
aerosolized so that they spread rapidly, infecting many by the
Rapid diagnosis by nucleic acid amplification test (NAAT) is
available for potential bioterrorism exposure. The definitive diagnosis
must be made in a laboratory equipped for and specialized
in handling this organism. Treatment is with a penicillin, fluoroquinolone,
erythromycin or tetracycline. Anthrax is prevented by
animal vaccination, sporicidal treatment of animal products and
vaccination of humans at high risk. Antibiotic prophylaxis is used
to prevent disease associated with known exposure. Vaccines are
available for military and related personal when biological agents
are a risk.
Bacillus cereus produces a heat-stable toxin. Typically, it multiplies
in parboiled rice and other contaminated food products, causing
a self-limiting food poisoning: vomiting occurs 6 h after exposure,
followed by diarrhoea (18 h).