Bacterial Infections

Mycoplasma and Ureaplasma species (mycoplasmas) are ubiquitous in nature and are commonly found in plants, animals, and humans. These bacteria contain the smallest amount of double-stranded DNA that is capable of producing a free-living microorganism; they measure between 0.15 and 0.3 um in diameter and = 2 um in length.

Infected humans are the only source of M pneumoniae organisms for transmission to new susceptible hosts. M pneumoniae is spread from one individual to another by respiratory droplets produced by coughing. Individuals at any age can be infected and develop disease, but those between the ages of 5 and 20 years are most often affected. M pneumoniae is a leading cause of pneumonia in school-aged children and young adults — especially those in military and college populations.

The syndrome of relapsing fever consists of two clinical entities: epidemic relapsing fever caused by Borrelia recurrentis (LBRF) and transmitted by the human body louse and endemic relapsing fever caused by Borrelia spp. (TBRF) and transmitted by arthropods (Table 1). A. Epidemiology. 1. Louse-borne epidemic relapsing fever (LBRF).

Lyme disease is a tick-borne illness caused by the spirochete B burgdorferi. Lyme disease can be divided into early disease (stage 1, EM), disseminated infection (stage 2), and late disease (stage 3, persistent infection). The first stage involves the skin, followed by stages 2 and 3, which often affect the skin, joints, CNS, and heart.

The term syphilis was first used in 1530 by the Italian physician Girolamo Fracastoro in his epic poem Syphilis Sive Morbus Gallicus. Much has been learned since then about this sexually transmitted disease caused by T pallidum.

Tertiary disease, usually seen 5-20 years after initial infection, traditionally includes cardiovascular syphilis, late benign (or gummatous) syphilis, and neurosyphilis (see Box 1). Fewer organisms are found in lesions during this stage. The incidence of cardiovascular involvement is probably underestimated, although clinically significant disease eventually develops in ~ 10% of all untreated patients.

The secondary stage of syphilis occasionally overlaps with the primary phase but usually begins ~ 6 weeks after resolution of the chancre; however, it can develop as late as 6 months after infection (see Box  1). Most patients have some degree of skin or mucocutaneous involvement. A faint and evanescent macular rash of the trunk and abdomen known as roseola syphilitica is sometimes seen initially.

The lesions of primary syphilis appear at the site of inoculation after an incubation period that is inversely proportional to the number of infecting organisms, usually 3 weeks (Box 1). The chancre is an ulcerative lesion that varies in size from several millimeters to 2 cm.

Originally thought to be fungi due to their hyphae-like appearance, they are now recognized as bacteria based on their cell wall components, reproduction by fission without sporulation or budding, inhibition by antibacterial agents, and molecular phylogenetic analysis. The actinomycete chromosomes contain a high content of guanosine and cytosine.

Disease occurs when mechanical insult disrupts the mucosal barrier or organisms gain access to privileged sites. For example, actinomycosis commonly occurs after dental procedures, trauma, surgery, or aspiration. Actinomyces israelii causes the majority of human disease owing to this genus, but other species, including Actinomyces naeslundii, Actinomyces viscosus, Actinomyces enksonii, Actinomyces odontolyticus, and Actinomyces meyeri have also been implicated. Actinomycosis is threefold more common in men than women.