Abstract
The effect of temperature fluctuation is an important factor in bacterial growth especially for pathogens such as the staphylococci that have to remain viable during potentially harsh and prolonged transfer conditions between hosts. The aim of this study was to investigate the response of S. aureus, S. epidermidis, and S. lugdunensis when exposed to low temperature (4°C) for prolonged periods, and how this factor affected their subsequent growth, colony morphology, cellular ultra-structure, and amino acid composition in the non-cytoplasmic hydrolysate fraction. Clinical isolates were grown under optimal conditions and then subjected to 4°C conditions for a period of 8 wks. Cold-stressed and reference control samples were assessed under transmission electron microscopy (TEM) to identify potential ultra-structural changes. To determine changes in amino acid composition, cells were fractured to remove the lipid and cytoplasmic components and the remaining structural components were hydrolysed. Amino acid profiles for the hydrolysis fraction were then analysed for changes by using principal component analysis (PCA). Exposure of the three staphylococci to prolonged low temperature stress resulted in the formation of increasing proportions of small colony variant (SCV) phenotypes. TEM revealed that SCV cells had significantly thicker and more diffuse cell-walls than their corresponding WT samples for both S. aureus and S. epidermidis, but the changes were not significant for S. lugdunensis. Substantial species-specific alterations in the amino acid composition of the structural hydrolysate fraction were also observed in the cold-treated cells. The data indicated that the staphylococci responded over prolonged periods of cold-stress treatment by transforming into SCV populations. The observed ultra-structural and amino acid changes were proposed to represent response mechanisms for staphylococcal survival amidst hostile conditions, thus maintaining the viability of the species until favourable conditions arise again.
Highlights
While staphylococci belong to the common flora of the skin and mucous membranes, they can quickly become opportunistic pathogens when the host’s immune system is breached to cause an array of diseases
Broth cultures of S. aureus S. epidermidis and S. lugdunensis exposed for extended periods to temperatures of 4uC after initial growth at 37uC yielded a range of colony variants on subculture onto HBA plates with variations in size, pigmentation, and haemolytic activity
The ability to form small colony variants (SCV) in response to 4uC stress was observed in all three staphylococcal species, their frequencies within the populations differed between species, and their abundances were associated with the duration of stress
Summary
While staphylococci belong to the common flora of the skin and mucous membranes, they can quickly become opportunistic pathogens when the host’s immune system is breached to cause an array of diseases. Exposures of staphylococci to antibiotics have been shown to be one cause for the formation of small-colony variants (SCVs) These variants represent sub-populations of bacteria that exhibit atypical growth features from those seen in their wild-type (WT) counterparts. SCVs have been associated with chronic persistent infections of the skeletal system, the heart and lungs, and other organ sites, when indwelling medical devices were used [7]. Treatment of such infections has become a challenge since SCVs were shown to be less susceptible to several antibiotics, and these phenotypes persist better intracellularly within host cells [8]. Further molecular studies have provided evidence that the clinical SCVs were different phenotypes compared to their WT parents but were capable of reversion to the WT form [16,17] suggesting that the phenotypic change involved significant alterations in metabolic homeostasis in the SCV
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