Small colony variants- a short introduction

I will discuss SCVs and their role in chronic infections in the next few blogposts. Below are the first few introductory paragraphs from the chapter on SCVs in my book.

Small colony variants (SCVs) constitute a naturally occurring, slow-growing subpopulation of bacteria that form small colonies (less than one-tenth of the size of parent colonies) on solid media (Proctor et al. 2006). SCVs were first described around 100 years ago and since then have been reported in a wide range of bacterial genera and species. However, they have been most extensively studied for Staphylococcus aureus (Proctor et al. 2006). The major characteristics of SCVs include slow growth rate, lack of pigmentation, reduced hemolytic activity, reduced coagulase activity, increased antibiotic resistance especially to aminoglycosides, reduced carbohydrate utilization and low virulence with reduced production of virulence factors.  Since the growth rate of SCVs is approximately nine times lower than the parent strains, they require a longer incubation time (48-72 h) to form pinpoint colonies on agar. The slow growth rate and formation of small colonies is often due to the inability of the bacteria to synthesize certain substances required for their growth; thus, supplementation of these substances in the growth medium can result in a normal growth rate (Proctor et al. 2006). Most of the SCVs reported are auxotrophic for hemin, menadione or thiamine, even though other compounds such as thymidine, unsaturated fatty acids  and CO₂ may also stimulate the growth of SCVs (Proctor et al. 2006).

They are implicated in a number of chronic infections, especially cystic fibrosis (CF) and osteomyelitis. SCVs are difficult to recognize using routine laboratory biochemical tests because of their atypical morphological and physiological features, thus presenting a challenge to clinical microbiologists.

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.