Mycoplasmas species - Overview

Overview

Pathogens belong to the Mycoplasma and Ureaplasma genera. They cause many diseases, especially respiratory diseases of farm animals including Contagious Bovine Pleuropneumonia. They can be involved in Mastitis and conjunctivitis in cattle and are implicated in respiratory and urinary tract diseases in dogs and cats. There are non-pathogenic mycoplasmas present in the rumen that live on the mucous membranes of the oronasal cavity, conjunctiva and intestines. Stress factors and concurrent disease may predispose to tissue invasion. Mycoplasmas are usually host-specific and have limited survival in the environment.

Characteristics

The Mycoplasmas are the smallest, free-living prokaryotic organisms. They are pleomorphic organisms and have no peptidoglycan cell wall. They are susceptible to dessication, heat and disinfectants and require enriched media containing animal protein, sterol and a source of DNA for growth. Their colonies have a fried egg appearance. Most are facultative anaerobes.

Pathogenesis and pathogenicity

Mycoplasmas adhere to host cells and produce toxins. Some adhere to neutrophils and macrophages and prevent phagocytosis. Mycoplasmas induce proliferation of macrophages and monocytes, and the release of cytokines such as TNF and interleukins. They cause damage to cilia in the respiratory tract leading to pneumonia. Molecular mimicry allows some mycoplasmas to avoid the host immune response and may initiate immune-mediated disease.

Diagnosis

Samples can be tested for the presence of mycoplasmas by fluorescent antibody techniques, peroxidase techniques and PCR. Biochemical profiles such as urease production can be used for identification. Ureaplasmas produce urease, whereas Mycoplasmas do not metabolise urea. Serolgy is required for specific identification including complement fixation tests, ELISA, agglutination tests and haemagglutination-inhibition tests. Growth inhibition tests using specific antisera can be used as well as fluorescent antibody tests.