Abstract
BackgroundSmall colony variants (SCVs), constituting a slow-growing subpopulation of bacteria that facilitates persistence in lethal environmental conditions, are able to revert to the phenotype of rapid growth for further proliferation and transmission. Salmonella enterica serotype Typhimurium is one of the most important foodborne pathogens. This study investigated the genetic mechanisms how SCVs induced by streptomycin reverted to the fast-growing phenotype and the phenotypic changes of SCVs among their complete life cycle in S.Typhimurium.MethodsSalmonella Typhimurium SCVs were obtained by streptomycin treatment and their revertants were collected in the absence of antibiotics. The fitness, antimicrobial susceptibility, biofilm formation, and the biofilm-related genes expression were analyzed in comparison to their wild type strain, and the whole genome sequencing was performed to identify the genetic changes in the life cycle of S. Typhimurium SCVs.ResultsSmall colony variants were characterized by an increased antimicrobial resistance to streptomycin (64-fold), imipenem (twofold), and gentamicin (fourfold). A significant increase in biofilm production with higher expression of csgB was observed in SCVs (P < 0.01). The genetic alterations of all SCVs occurred in ubiE gene (coenzyme Q8 and menaquinone synthesis) with frameshift mutations. However, all fast-growing revertants again lost the trait of increased biofilm production (P > 0.05), in which two modes of the genetic changes for reversing to the rapidly growing form were observed: four revertants harbored a secondary mutation in ubiE, which reinstated most of the amino acid sequence of the ubiE, and other four revertants harbored a mutation in prfB.ConclusionsSalmonella Typhimurium could switch to the phenotype of SCVs under the treatment of streptomycin by a mutation in ubiE, partially combined with increased production of biofilm, and these SCVs could escape from growth restriction by a compensatory mutation in prfB or a new mutation in ubiE. These findings may contribute to establishing phenotype-directed treatments against SCVs of S.Typhimurium.Electronic supplementary materialThe online version of this article (doi:10.1186/s12941-016-0151-3) contains supplementary material, which is available to authorized users.
Highlights
Small colony variants (SCVs), constituting a slow-growing subpopulation of bacteria that facilitates persistence in lethal environmental conditions, are able to revert to the phenotype of rapid growth for further proliferation and transmission
Small colony variants (SCVs), an aberrant form of bacterial phonotype, constitute a slow-growing subpopulation typically characterized with reduced metabolic activity and increased resistance to antibiotics [1, 2], thereby protecting bacterial cells against some non-optimal and lethal environmental conditions [3]
One class of fast-growing strains was observed on antibioticfree Mueller– Hinton agar (MHA) agar after 5-day incubation at room temperature, and eight revertants reverted from strain 1024-4 or 1024-5 were collected
Summary
Small colony variants (SCVs), constituting a slow-growing subpopulation of bacteria that facilitates persistence in lethal environmental conditions, are able to revert to the phenotype of rapid growth for further proliferation and transmission. STR, which was first used in plant agriculture in the 1950’s, remains to be an important antibiotic for bacterial disease control in both animals and plants, especially in developing countries [10, 11]. Agricultural products, such as fruits, vegetables, livestock and poultry meats were found to be the most common sources of Salmonella enterica infections [12,13,14]. SCVs may be induced by exposure to STR in agricultural products in these countries
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