Chapter 92: Anaerobic Infections

DEFINITIONS

• Anaerobic bacteria: require reduced oxygen tension for growth; do not grow on the surface of solid media in 10% CO₂ in air

• Microaerophilic bacteria: grow in an atmosphere of 10% CO₂ in air or under anaerobic or aerobic conditions, but grow best if only a small amount of atmospheric oxygen is present

• Facultative bacteria: grow in the presence or absence of oxygen

TETANUS

Microbiology, Epidemiology, and Pathogenesis

Tetanus is characterized by increased muscle tone and spasms caused by tetanospasmin (“tetanus toxin”), a toxin produced by Clostridium tetani.

• C. tetani is a spore-forming, anaerobic gram-positive rod that is ubiquitous in soil and whose spores are highly resilient.

• Tetanus is a rare disease in the developed world: only 231 cases were reported in the United States between 2001 and 2008. Most cases occur in incompletely vaccinated or unvaccinated individuals.

• After spores contaminate wounds (typically puncture wounds or, in the case of neonates, the umbilical stump) and reach a suitable anaerobic environment (e.g., devitalized tissue), organisms proliferate and release toxin.

  • – The toxin blocks release of inhibitory neurotransmitters (glycine and γ-aminobutyric acid) in presynaptic terminals, and rigidity results from an increased resting firing rate of the α-motor neurons.

  • – A toxin dose as low as 2.5 ng/kg can be fatal.

Clinical Manifestations

C. tetani can cause a usually mild local disease confined to the muscles near the wound or a more severe generalized disease (e.g., neonatal disease).

• If the cranial nerves are involved in localized cephalic tetanus, the pt may aspirate or develop airway obstruction due to pharyngeal or laryngeal muscle spasm. This situation is associated with a poor prognosis.

• The early signs and symptoms of generalized tetanus often include trismus (lockjaw), muscle pain and stiffness, back pain, and difficulty swallowing. As the disease progresses, painful muscle spasms develop and can sometimes be strong enough to cause crush fractures.

  • – Without ventilatory support, respiratory failure is the most common cause of death in tetanus.

  • – Autonomic disturbance (e.g., labile blood pressure; GI stasis; increased tracheal secretions; acute, high-output renal failure) is maximal during the second week of severe tetanus, and death due to cardiovascular events becomes the major risk.

Diagnosis

The diagnosis is made on clinical grounds. Culture of C. tetani from a wound provides supportive evidence.