Modification of bactericidal fatty acids by an enzyme of Staphylococcus aureus

Lipase-mediated detoxification of host-derived antimicrobial fatty acids by Staphylococcus aureus

Long-chain fatty acids with antimicrobial properties are abundant on the skin and mucosal surfaces, where they are essential to restrict the proliferation of opportunistic pathogens such as Staphylococcus aureus. These antimicrobial fatty acids (AFAs) elicit bacterial adaptation strategies, which have yet to be fully elucidated. Characterizing the pervasive mechanisms used by S. aureus to resist AFAs could open new avenues to prevent pathogen colonization. Here, we identify the S. aureus lipase Lip2 as a novel resistance factor against AFAs. Lip2 detoxifies AFAs via esterification with cholesterol. This is reminiscent of the activity of the fatty acid-modifying enzyme (FAME), whose identity has remained elusive for over three decades. In vitro, Lip2-dependent AFA-detoxification was apparent during planktonic growth and biofilm formation. Our genomic analysis revealed that prophage-mediated inactivation of Lip2 was rare in blood, nose, and skin strains, suggesting a particularly important role of Lip2 for host - microbe interactions. In a mouse model of S. aureus skin colonization, bacteria were protected from sapienic acid (a human-specific AFA) in a cholesterol- and lipase-dependent manner. These results suggest Lip2 is the long-sought FAME that exquisitely manipulates environmental lipids to promote bacterial growth in otherwise inhospitable niches.

Gentamicin resistance to Escherichia coli related to fatty acid metabolism based on transcriptome analysis

Antibiotic overuse and misuse have promoted the emergence and spread of antibiotic-resistant bacteria. Increasing bacterial resistance to antibiotics is a major healthcare problem, necessitating elucidation of antibiotic resistance mechanisms. In this study, we explored the mechanism of gentamicin resistance by comparing the transcriptomes of antibiotic-sensitive and -resistant Escherichia coli. A total of 410 differentially expressed genes were identified, of which 233 (56.83%) were up-regulated and 177 (43.17%) were down-regulated in the resistant strain compared with the sensitive strain. Gene Ontology (GO) analysis classifies differential gene expression into three main categories: biological processes, cellular components, and molecular functions. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that the up-regulated genes were enriched in eight metabolic pathways, including fatty acid metabolism, which suggests that fatty acid metabolism may be involved in the development of gentamicin resistance in E. coli. This was demonstrated by measuring the acetyl-CoA carboxylase activity, plays a fundamental role in fatty acid metabolism, was increased in gentamicin-resistant E. coli. Treatment of fatty acid synthesis inhibitor, triclosan, promoted gentamicin-mediated killing efficacy to antibiotic-resistant bacteria. We also found that exogenous addition of oleic acid, which involved in fatty acid metabolism, reduced E. coli sensitivity to gentamicin. Overall, our results provide insight into the molecular mechanism of gentamicin resistance development in E. coli.

Bacterial adaptation strategies to host-derived fatty acids

Fatty acids (FAs) are potent antimicrobials which hold great promise as viable alternatives or complements to conventional antibiotics. Intriguingly, bacteria are well equipped to use environmental FAs as energy sources and/or building blocks for their membrane lipids. Furthermore, these microbes display a wide array of mechanisms to prevent or mitigate FA toxicity. In this review we discuss strategies that bacteria use to thrive despite extensive exposure to host-derived antimicrobial FAs. We also highlight the altered response of these FA-adapted bacteria to antibiotics. Given the ubiquitous nature of FAs in various host environments, deciphering bacterial adaptation strategies to FAs is of prime importance. This knowledge may pave the way for a rational design of FA-based combination therapies with antibiotics.

Staphylococcus aureus adapts to the host nutritional landscape to overcome tissue-specific branched-chain fatty acid requirement

During infection, pathogenic microbes adapt to the nutritional milieu of the host through metabolic reprogramming and nutrient scavenging. For the bacterial pathogen Staphylococcus aureus, virulence in diverse infection sites is driven by the ability to scavenge myriad host nutrients, including lipoic acid, a cofactor required for the function of several critical metabolic enzyme complexes. S. aureus shuttles lipoic acid between these enzyme complexes via the amidotransferase, LipL. Here, we find that acquisition of lipoic acid, or its attachment via LipL to enzyme complexes required for the generation of acetyl-CoA and branched-chain fatty acids, is essential for bacteremia, yet dispensable for skin infection in mice. A lipL mutant is auxotrophic for carboxylic acid precursors required for synthesis of branched-chain fatty acids, an essential component of staphylococcal membrane lipids and the agent of membrane fluidity. However, the skin is devoid of branched-chain fatty acids. We showed that S. aureus instead scavenges host-derived unsaturated fatty acids from the skin using the secreted lipase, Geh, and the unsaturated fatty acid-binding protein, FakB2. Moreover, murine infections demonstrated the relevance of host lipid assimilation to staphylococcal survival. Altogether, these studies provide insight into an adaptive trait that bypasses de novo lipid synthesis to facilitate S. aureus persistence during superficial infection. The findings also reinforce the inherent challenges associated with targeting bacterial lipogenesis as an antibacterial strategy and support simultaneous inhibition of host fatty acid salvage during treatment.

The Role of Macrophages in Staphylococcus aureus Infection

Staphylococcus aureus is a member of the human commensal microflora that exists, apparently benignly, at multiple sites on the host. However, as an opportunist pathogen it can also cause a range of serious diseases. This requires an ability to circumvent the innate immune system to establish an infection. Professional phagocytes, primarily macrophages and neutrophils, are key innate immune cells which interact with S. aureus, acting as gatekeepers to contain and resolve infection. Recent studies have highlighted the important roles of macrophages during S. aureus infections, using a wide array of killing mechanisms. In defense, S. aureus has evolved multiple strategies to survive within, manipulate and escape from macrophages, allowing them to not only subvert but also exploit this key element of our immune system. Macrophage-S. aureus interactions are multifaceted and have direct roles in infection outcome. In depth understanding of these host-pathogen interactions may be useful for future therapeutic developments. This review examines macrophage interactions with S. aureus throughout all stages of infection, with special emphasis on mechanisms that determine infection outcome.

Staphylococcus aureus Secreted Toxins and Extracellular Enzymes

Staphylococcus aureus is a formidable pathogen capable of causing infections in different sites of the body in a variety of vertebrate animals, including humans and livestock. A major contribution to the success of S. aureus as a pathogen is the plethora of virulence factors that manipulate the host's innate and adaptive immune responses. Many of these immune modulating virulence factors are secreted toxins, cofactors for activating host zymogens, and exoenzymes. Secreted toxins such as pore-forming toxins and superantigens are highly inflammatory and can cause leukocyte cell death by cytolysis and clonal deletion, respectively. Coagulases and staphylokinases are cofactors that hijack the host's coagulation system. Exoenzymes, including nucleases and proteases, cleave and inactivate various immune defense and surveillance molecules, such as complement factors, antimicrobial peptides, and surface receptors that are important for leukocyte chemotaxis. Additionally, some of these secreted toxins and exoenzymes can cause disruption of endothelial and epithelial barriers through cell lysis and cleavage of junction proteins. A unique feature when examining the repertoire of S. aureus secreted virulence factors is the apparent functional redundancy exhibited by the majority of the toxins and exoenzymes. However, closer examination of each virulence factor revealed that each has unique properties that have important functional consequences. This chapter provides a brief overview of our current understanding of the major secreted virulence factors critical for S. aureus pathogenesis.

Characterization of fatty acid modifying enzyme activity in staphylococcal mastitis isolates and other bacteria

Background

Fatty acid modifying enzyme (FAME) has been shown to modify free fatty acids to alleviate their bactericidal effect by esterifying fatty acids to cholesterol or alcohols. Although it has been shown in previous studies that FAME is required for Staphylococcus aureus survival in skin abscesses, FAME is poorly studied compared to other virulence factors. FAME activity had also been detected in coagulase-negative staphylococci (CNS). However, FAME activity was only surveyed after a bacterial culture was grown for 24 h. Therefore if FAME activity was earlier in the growth phase, it would not have been detected by the assay and those strains would have been labeled as FAME negative.

Results

Fifty CNS bovine mastitis isolates and several S. aureus, Escherichia coli, and Streptococcus uberis strains were assayed for FAME activity over 24 h. FAME activity was detected in 54% of CNS and 80% S. aureus strains surveyed but none in E. coli or S. uberis. While some CNS strains produced FAME activity comparable to the lab strain of S. aureus, the pattern of FAME activity varied among strains and across species of staphylococci. All CNS that produced FAME activity also exhibited lipase activity. Lipase activity relative to colony forming units of these CNS decreased over the 24 h growth period. No relationship was observed between somatic cell count in the milk and FAME activity in CNS.

Conclusions

Some staphylococcal species surveyed produced FAME activity, but E. coli and S. uberis strains did not. All FAME producing CNS exhibited lipase activity which may indicate that both these enzymes work in concert to alter fatty acids in the bacterial environment.

Characterisation of a cell wall-anchored protein of Staphylococcus saprophyticus associated with linoleic acid resistance

Background

The Gram-positive bacterium Staphylococcus saprophyticus is the second most frequent causative agent of community-acquired urinary tract infections (UTI), accounting for up to 20% of cases. A common feature of staphylococci is colonisation of the human skin. This involves survival against innate immune defenses including antibacterial unsaturated free fatty acids such as linoleic acid which act by disrupting bacterial cell membranes. Indeed, S. saprophyticus UTI is usually preceded by perineal skin colonisation.

Results

In this study we identified a previously undescribed 73.5 kDa cell wall-anchored protein of S. saprophyticus, encoded on plasmid pSSAP2 of strain MS1146, which we termed S. saprophyticus surface protein F (SssF). The sssF gene is highly prevalent in S. saprophyticus clinical isolates and we demonstrate that the SssF protein is expressed at the cell surface. However, unlike all other characterised cell wall-anchored proteins of S. saprophyticus, we were unable to demonstrate a role for SssF in adhesion. SssF shares moderate sequence identity to a surface protein of Staphylococcus aureus (SasF) recently shown to be an important mediator of linoleic acid resistance. Using a heterologous complementation approach in a S. aureus sasF null genetic background, we demonstrate that SssF is associated with resistance to linoleic acid. We also show that S. saprophyticus strains lacking sssF are more sensitive to linoleic acid than those that possess it. Every staphylococcal genome sequenced to date encodes SssF and SasF homologues. Proteins in this family share similar predicted secondary structures consisting almost exclusively of α-helices in a probable coiled-coil formation.

Conclusions

Our data indicate that SssF is a newly described and highly prevalent surface-localised protein of S. saprophyticus that contributes to resistance against the antibacterial effects of linoleic acid. SssF is a member of a protein family widely disseminated throughout the staphylococci.

The Staphylococcus aureus response to unsaturated long chain free fatty acids: survival mechanisms and virulence implications

Staphylococcus aureus is an important human commensal and opportunistic pathogen responsible for a wide range of infections. Long chain unsaturated free fatty acids represent a barrier to colonisation and infection by S. aureus and act as an antimicrobial component of the innate immune system where they are found on epithelial surfaces and in abscesses. Despite many contradictory reports, the precise anti-staphylococcal mode of action of free fatty acids remains undetermined. In this study, transcriptional (microarrays and qRT-PCR) and translational (proteomics) analyses were applied to ascertain the response of S. aureus to a range of free fatty acids. An increase in expression of the σ^B and CtsR stress response regulons was observed. This included increased expression of genes associated with staphyloxanthin synthesis, which has been linked to membrane stabilisation. Similarly, up-regulation of genes involved in capsule formation was recorded as were significant changes in the expression of genes associated with peptidoglycan synthesis and regulation. Overall, alterations were recorded predominantly in pathways involved in cellular energetics. In addition, sensitivity to linoleic acid of a range of defined (sigB, arcA, sasF, sarA, agr, crtM) and transposon-derived mutants (vraE, SAR2632) was determined. Taken together, these data indicate a common mode of action for long chain unsaturated fatty acids that involves disruption of the cell membrane, leading to interference with energy production within the bacterial cell. Contrary to data reported for other strains, the clinically important EMRSA-16 strain MRSA252 used in this study showed an increase in expression of the important virulence regulator RNAIII following all of the treatment conditions tested. An adaptive response by S. aureus of reducing cell surface hydrophobicity was also observed. Two fatty acid sensitive mutants created during this study were also shown to diplay altered pathogenesis as assessed by a murine arthritis model. Differences in the prevalence and clinical importance of S. aureus strains might partly be explained by their responses to antimicrobial fatty acids.

Antibiotic-based selection for bacterial genes that are specifically induced during infection of a host

We have recently described a genetic system, termed in vivo expression technology (IVET), that uses an animal as a selective medium to identify genes that pathogenic bacteria specifically express when infecting host tissues. Here, the potential utility of the IVET approach has been expanded with the development of a transcriptional-fusion vector, pIVET8, which uses antibiotics resistance as the basis for selection in host tissues. pIVET8 contains promoterless chloramphenicol acetyltransferase (cat) and lacZY genes. A pool of Salmonella typhimurium clones carrying random cat-lac transcriptional fusions, produced with pIVET8, was used to infect BALB/c mice that were subsequently treated with intraperitoneal injections of chloramphenicol. Strains that survived the selection by expressing the cat gene in the animal were then screened for those that had low-level lacZY expression on laboratory medium. These strains carry operon fusions to genes that are specifically induced in vivo (ivi genes). One of the ivi genes identified (fadB) encodes an enzyme involved in fatty acid oxidation, suggesting that this enzyme might contribute to the metabolism of bactericidal or proinflammatory host fatty acids. The pIVET8-based selection system was also used to identify S. typhimurium genes that are induced in cultured macrophages. The nature of ivi gene products will provide a more complete understanding of the metabolic, physiological, and genetic factors that contribute to the virulence of microbial pathogens.

Analyses of skin surface lipid in patients with microbially associated skin disease

The composition of the total skin surface lipid, free fatty acids and fatty acid of triglyceride was used in discriminant analyses to classify patients with severe acne vulgaris (AV), pityriasis versicolor (PV), seborrhoeic dermatitis (SD) and atopic dermatitis (AD). It was not possible to discriminate between the PV and SD patients but all the lipid groups gave a satisfactory separation between AV, PV + SD and AD patients. A similar separation was obtained when the sex of the patient was studied but not when both sex and disease were taken into account. The results suggest that there are fundamental differences in the composition of skin lipid between patients with microbially associated diseases, but the precise role of the organisms has not been ascertained.