Kenny et al.  observed that sasF was the most upregulated gene in S. aureus MRSA252 microarray and qRT-PCR experiments upon challenge with linoleic acid. The protective function of SasF was apparent when examined in a linoleic acid emulsion agar plate-based bacterial survival assay. Our hypothesis focused on the possibility that SssF possessed a similar function to SasF, but no linoleic acid resistance phenotype for SssF was observed in the S. saprophyticus MS1146 genetic background. Using the linoleic acid emulsion agar plate bacterial survival assay in the presence 0.85 M NaCl, we observed a higher survival amongst S. Citarinostat saprophyticus
strains that harbour the sssF gene than those that lack sssF. We then successfully expressed SssF heterologously in a S. aureus SH1000sasF host and demonstrated restored resistance to linoleic acid. We found S. saprophyticus MS1146 to be intrinsically more resistant to linoleic acid than S. aureus SH1000. This remains to be explored but could be due to a number of species/strain specific factors including the action of redundant S. saprophyticus MS1146 resistance mechanisms or variations in surface
components such as capsule or teichoic acids. We found that the survival of S. aureus SH1000 and its derivatives was markedly selleck chemicals llc increased in the presence of linoleic acid at pH 6.0 compared to pH 7.4. This result is consistent with previous studies of the staphylococcal fatty acid modifying enzyme (FAME), an unidentified but partially characterised protein secreted by most staphylococci selleck screening library which detoxifies free fatty acids by esterifying them to an alcohol
[34, 35]. The FAME of S. aureus and S. epidermidis demonstrate optimal activity at pH 6.0, and have little activity at pH 7.4 [35, 36]. This is congruent with human skin having a slightly acidic pH of 5.5-6 . RP-HPLC experiments using linoleic acid and crude protein extracts demonstrated that SssF activity is distinct from FAME activity (data not shown). Other antimicrobial fatty acids such as sapienic acid have yet to be examined as substrates for SssF or SasF. We hypothesise that some or all of the other uncharacterised SssF-like proteins exhibit fatty acid resistance activity, but this remains to be demonstrated experimentally. There are precedents for bacterial surface structures that provide protection against bactericidal free fatty acids. Gram-positive bacterial cell wall teichoic acids provide protection against free fatty acid mediated killing of S. aureus . The IsdA protein of S. aureus reduces bacterial hydrophobicity when expressed at the cell surface under the cue of iron starvation to resist fatty acid membrane attack and also promotes fatty acid resistance of S. aureus in a volunteer human skin survival model . Our studies however found that expression of SssF does not influence cell surface hydrophobicity of S. saprophyticus, and this corresponds with matching data for SasF and S. aureus .