Small colony variants- a survival strategy of bacteria: literature review

The formation of small colony variants (SCVs) is considered as a survival strategy of bacteria to evade antibiotic killing. In the presence of antibiotics, especially aminoglycosides, SCVs may survive due to reduced uptake of the antibiotic when most of the bacteria get killed. Thus, within a host, antibiotic pressure may select electron transport deficient mutants which may survive intracellularly due to low levels of free hemin and menadione within the host cell (McNamara and Proctor 2000). Since the intracellular milieu protects bacteria from antibodies, complements and many antibiotics, increased intracellular persistence of SCVs could be a survival strategy of S. aureus. However, once the antibiotic is removed, SCVs may revert to normal phenotype resulting in chronic infections.

In many clinical cases of chronic infections, both the SCVs and large colony types have been isolated. Researchers have shown that both colony types are clonal indicating a common origin. Isolation of SCVs from osteomyelitis and cystic fibrosis supports a pathogenic role for SCVs in such diseases (Proctor et al. 2006). SCVs have also been isolated from device-related infections, persistent wound infections and persistent bovine mastitis.

SCVs are implicated not only in chronic infections, but also in fatal infections. For example, a fatal infection due to SCVs of methicillin-resistant S. aureus in a patient with AIDS has been reported (Seifert et al. 1999). Similarly, SCVs of the Burkholderia cepacia complex has been reported to be responsible for the fatal outcome of lung transplantation in CF patients (Haussler et al. 2003).

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Proctor et al. (2006). Small colony variants: a pathogenic form of bacteria that facilitates persistent and recurrent infections. Nat Rev Microbiol 4(4), 295-305.

McNamara, P. J., and Proctor, R. A. (2000). Staphylococcus aureus small colony variants, electron transport and persistent infections. Int J Antimicrob Agents 14(2), 117-22.

Seifert et al. (1999). Fatal case due to methicillin-resistant Staphylococcus aureus small colony variants in an AIDS patient. Emerg Infect Dis 5(3), 450-3.

Haussler et al. (2003). Fatal outcome of lung transplantation in cystic fibrosis patients due to small-colony variants of the Burkholderia cepacia complex. Eur J Clin Microbiol Infect Dis 22(4), 249-53.