Small colony variants (SCVs) are slow-growing bacteria with distinctive phenotypic and pathogenic traits. SCVs are often less susceptible to antibiotics and can cause latent or recurrent infections. During sub-culturing of P. aeruginosa on gentamicin, one SCV was isolated, which showed resistance to the antibiotic, a delayed lag-phase in LB or in casamino acid medium, and a defective motility. It also produced low levels of C4-acylhomoserine lactone and of the 2-alkyl-4-quinolones signal quorum sensing molecules (AQs) compared to the wild-type. Virulence factors, such as the siderophore pyoverdine, elastase and the redox-active phenazine pigment pyocyanin were also reduced. On excised Roman lettuce (Lactuca sativa) leaves the SCV caused strongly reduced necrosis compared to the wild-type. The SCV caused also reduced mortality in Drosophila. A proteome comparison revealed a highly reduced production of the extracellular chitinase ChiC and the flagellar capping protein FliD. Conversely, production of the two-component response regulator PhoP was much higher in the SCV. The transcriptome analysis revealed a down-regulation of 74 genes, including those for the synthesis of AQs, and other QS-regulated genes such as the elastase gene lasA or phenazine biosynthesis genes, in agreement with the phenotypic characterization results. Hundred sixty genes were upregulated in the SCV mutant, among them the antibiotic resistance genes mexR, mexY, mexX, and mexZ and LPS modification genes PA3552-PA3559, and the phoP and oprH genes, confirming the proteome results.

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