High sodium chloride concentrations inhibit staphylococcal enterotoxin C gene (sec) expression at the level of sec mRNA

Stress Lowers Staphylococcal Enterotoxin C Production Independently of Agr, SarA, and SigB

Staphylococcal enterotoxin C (SEC) can cause staphylococcal food poisoning, one of the most prevalent foodborne intoxications. It is produced by Staphylococcus aureus during growth in the food matrix. While the surrounding bacteria in food matrices usually repress the growth of S.aureus, the organism possesses a remarkable growth advantage under stressful conditions encountered in many foods. Examples for such food matrices are pastry and bakery products with their high sugar content that lowers water availability. While S. aureus can still grow in these challenging environments, it remains unclear how these conditions affect SEC expression. Here, the influence of 30% glucose on sec mRNA in a qPCR assay and SEC protein expression was investigated for the first time in an ELISA. In addition, regulatory knockout mutants Δagr, ΔsarA, and ΔsigB were generated to investigate regulatory gene elements in glucose stress. In five out of seven strains, glucose stress led to a pronounced decrease in sec mRNA transcription and SEC protein levels were substantially lower under glucose stress. It could be shown that key regulatory elements Δagr, ΔsarA, and ΔsigB in strain SAI48 did not contribute to the pronounced downregulation under glucose stress. Based on these findings, glucose effectively lowers SEC synthesis in the food matrix. However, the mechanism by which it acts on toxin expression and regulatory elements in S. aureus remains unclear. Future studies on other regulatory elements and transcriptomics may shed light on the mechanisms.

Mild NaCl Stress Influences Staphylococcal Enterotoxin C Transcription in a Time-Dependent Manner and Reduces Protein Expression

Enterotoxins (SEs) produced by Staphylococcus aureus are the cause of serious food intoxications. Staphylococcal enterotoxin C (SEC) is one of the main contributors, as it is often highly expressed. S. aureus possesses a competitive growth advantage over accompanying bacterial flora under stress conditions encountered in foods, such as high NaCl concentrations. However, the influence of NaCl as an external stressor on SEC expression is still unclear. We investigated the influence of 4.5% NaCl on sec mRNA and SEC protein levels. A qRT-PCR assay revealed that NaCl stress leads to time-dependently decreased or elevated sec mRNA levels for most strains. SEC protein levels were generally decreased under NaCl stress. Our findings suggest that NaCl stress lowers overall SEC concentration and time-dependently affects sec mRNA levels.

The effect of sodium chloride and temperature on the levels of transcriptional expression of staphylococcal enterotoxin genes in Staphylococcus aureus isolates from broiler carcasses

Staphylococcus aureus is one of the most common pathogens associated with food poisoning, which is caused by the ingestion of food contaminated with staphylococcal enterotoxins (SE). Our study aims at evaluating the occurrence and expression of five SE genes (sea, seb, sec, sed, and see) in S. aureus previously isolated from broiler carcasses. Besides that, it also presents an in vitro analysis of the effects of sodium chloride and temperature on the levels of transcriptional expression. A total of 30 S. aureus isolates were investigated for the presence of SEs by PCR assay. The expression level and the effects of sodium chloride (2.5% NaCl), as well as temperature (8 ºC and 12 ºC), on the transcriptional expression, were evaluated by a quantitative reverse transcription PCR (RT-qPCR). Twelve isolates carried at least one of the SE genes. Among them, five representative isolates presented transcriptional expression for at least one gene. Both sodium chloride and low temperatures interfered with the expression of the SE genes, decreasing their values. However, one isolate displayed relative expression 2.25 times higher for sed gene than S. aureus FRI 361 in optimal conditions (p < 0.05), demonstrating their toxigenic potential even under salt stress. There was no evidence of enterotoxin gene expression at 8 ºC.

Staphylococcal Enterotoxin C-An Update on SEC Variants, Their Structure and Properties, and Their Role in Foodborne Intoxications

Staphylococcal enterotoxins are the most common cause of foodborne intoxications (staphylococcal food poisoning) and cause a wide range of diseases. With at least six variants staphylococcal enterotoxin C (SEC) stands out as particularly diverse amongst the 25 known staphylococcal enterotoxins. Some variants present unique and even host-specific features. Here, we review the role of SEC in human and animal health with a particular focus on its role as a causative agent for foodborne intoxications. We highlight structural features unique to SEC and its variants, particularly, the emetic and superantigen activity, as well as the roles of SEC in mastitis and in dairy products. Information about the genetic organization as well as regulatory mechanisms including the accessory gene regulator and food-related stressors are provided.

Silence as a way of niche adaptation: mecC-MRSA with variations in the accessory gene regulator (agr) functionality express kaleidoscopic phenotypes

Functionality of the accessory gene regulator (agr) quorum sensing system is an important factor promoting either acute or chronic infections by the notorious opportunistic human and veterinary pathogen Staphylococcus aureus. Spontaneous alterations of the agr system are known to frequently occur in human healthcare-associated S. aureus lineages. However, data on agr integrity and function are sparse regarding other major clonal lineages. Here we report on the agr system functionality and activity level in mecC-carrying methicillin resistant S. aureus (MRSA) of various animal origins (n = 33) obtained in Europe as well as in closely related human isolates (n = 12). Whole genome analysis assigned all isolates to four clonal complexes (CC) with distinct agr types (CC599 agr I, CC49 agr II, CC130 agr III and CC1943 agr IV). Agr functionality was assessed by a combination of phenotypic assays and proteome analysis. In each CC, isolates with varying agr activity levels were detected, including the presence of completely non-functional variants. Genomic comparison of the agr I-IV encoding regions associated these phenotypic differences with variations in the agrA and agrC genes. The genomic changes were detected independently in divergent lineages, suggesting that agr variation might foster viability and adaptation of emerging MRSA lineages to distinct ecological niches.

A Functional ClpXP Protease is Required for Induction of the Accessory Toxin Genes, tst, sed, and sec

Staphylococcal toxic shock syndrome is a potentially lethal illness attributed to superantigens produced by Staphylococcus aureus, in particular toxic shock syndrome toxin 1 (TSST-1), but staphylococcal enterotoxins (SEs) are also implicated. The genes encoding these important toxins are carried on mobile genetic elements, and the regulatory networks controlling expression of these toxins remain relatively unexplored. We show here that the highly conserved ClpXP protease stimulates transcription of tst (TSST-1), sec (SEC), and sed (SED) genes in the prototypical strains, SA564 and RN4282. In the wild-type cells, the post-exponential upregulation of toxin gene transcription was proposed to occur via RNAIII-mediated downregulation of the Rot repressor. Contradictive to this model, we showed that the post-exponential induction of tst, sed, and sec transcription did not occur in cells devoid of ClpXP activity, despite the Rot level being diminished. To identify transcriptional regulators with a changed expression in cells devoid of ClpXP activity, RNA sequencing was performed. The RNAseq analysis revealed a number of global virulence regulators that might act downstream of ClpXP, to control expression of tst and other virulence genes. Collectively, the results extend our understanding of the complex transcriptional regulation of the tst, sed, and sec genes.

Basis of Virulence in Enterotoxin-Mediated Staphylococcal Food Poisoning

The Staphylococcus aureus enterotoxins are a superfamily of secreted virulence factors that share structural and functional similarities and possess potent superantigenic activity causing disruptions in adaptive immunity. The enterotoxins can be separated into two groups; the classical (SEA-SEE) and the newer (SEG-SElY and counting) enterotoxin groups. Many members from both these groups contribute to the pathogenesis of several serious human diseases, including toxic shock syndrome, pneumonia, and sepsis-related infections. Additionally, many members demonstrate emetic activity and are frequently responsible for food poisoning outbreaks. Due to their robust tolerance to denaturing, the enterotoxins retain activity in food contaminated previously with S. aureus. The genes encoding the enterotoxins are found mostly on a variety of different mobile genetic elements. Therefore, the presence of enterotoxins can vary widely among different S. aureus isolates. Additionally, the enterotoxins are regulated by multiple, and often overlapping, regulatory pathways, which are influenced by environmental factors. In this review, we also will focus on the newer enterotoxins (SEG-SElY), which matter for the role of S. aureus as an enteropathogen, and summarize our current knowledge on their prevalence in recent food poisoning outbreaks. Finally, we will review the current literature regarding the key elements that govern the complex regulation of enterotoxins, the molecular mechanisms underlying their enterotoxigenic, superantigenic, and immunomodulatory functions, and discuss how these activities may collectively contribute to the overall manifestation of staphylococcal food poisoning.

Expression of Staphylococcal Enterotoxins under Stress Encountered during Food Production and Preservation

Staphylococcal food poisoning (SFP) is the most prevalent cause of food-borne intoxications worldwide. Consumption of enterotoxins preformed in food causes violent vomiting and can be fatal in children and the elderly. While being repressed by competing bacteria in most matrices, Staphylococcus aureus benefits from crucial competitive advantages in foods with high osmolarity or low pH. During recent years, the long-standing belief in the feasibility of assessing SFP risk based on colony-forming units of S. aureus present in food products has been disproven. Instead, researchers and food business operators are acutely aware of the imminent threat arising from unforeseeable enterotoxin production under stress conditions. This paradigm shift led to a variety of new publications enabling an improved understanding of enterotoxin expression under stress conditions encountered in food. The wealth of data provided by these studies is extremely diverse, as it is based on different methodological approaches, staphylococcal strains, stressors, and enterotoxins. Therefore, in this review, we aggregated and critically evaluated the complex findings of these studies, to provide readers with a current overview of the state of research in the field.

Sodium Chloride Does Not Ensure Microbiological Safety of Foods: Cases and Solutions

Addition of salt or salt-containing water to food is one of the oldest and most effective preservation methods in history; indeed, salt-cured foods are generally recognized as microbiologically safe due to their high salinity. However, a number of microbiological risks remain. The microbiological hazards and risks associated with salt-cured foods must be addressed more in-depth as they are likely to be underestimated by previous studies. This review examined a number of scientific reports and articles about the microbiological safety of salt-cured foods, which included salted, brined, pickled, and/or marinated vegetables, meat, and seafood. The following subjects are covered in order: (1) clinical cases and outbreaks attributed to salt-cured foods; (2) the prevalence of foodborne pathogens in such foods; (3) the molecular, physiological, and virulent responses of the pathogens to the presence of NaCl in both laboratory media and food matrices; (4) the survival and fate of microorganisms in salt-cured foods (in the presence/absence of additional processes); and (5) the interaction between NaCl and other stressors in food processes (e.g., acidification, antimicrobials, drying, and heating). The review provides a comprehensive overview of potentially hazardous pathogens associated with salt-cured foods and suggests further research into effective intervention techniques that will reduce their levels in the food chain.

Staphylococcal adaptation to diverse physiologic niches: an overview of transcriptomic and phenotypic changes in different biological environments

Host niches can differ strongly regarding, for example, oxygen tension, pH or nutrient availability. Staphylococcus aureus and other staphylococci are common colonizers of human epithelia as well as important human pathogens. The phenotypes that they show in different host environments, and the corresponding bacterial transcriptomes and proteomes, are currently under intense investigation. In this review, we examine the available literature describing staphylococcal phenotypes, such as expression of virulence factors, gross morphologic characteristics and growth patterns, in various physiological environments. Going forward, these studies will help researchers and clinicians to form an enhanced and more detailed picture of the interactions existing between the host and staphylococci as some of its most frequent colonizers and invaders.

Temporal expression of the staphylococcal enterotoxin D gene under NaCl stress conditions

Staphylococcus aureus is one of the most osmotolerant food-borne pathogens. While its growth is repressed by competing bacteria, the organism exhibits a growth advantage at increased salt concentrations. Staphylococcal enterotoxin D leads to vomiting and diarrhea upon ingestion. To date, the effect of NaCl on both sed expression and its regulatory control are unclear. We determined the impact of NaCl stress on sed expression and the influence of agr, sarA and sigB on sed expression under NaCl stress. The temporal expression of sed in LB and LB with 4.5% NaCl was compared, as well as sed expression of wild-type (wt) strains and isogenic Δagr, ΔsarA and ΔsigB mutants. In general, NaCl stress led to decreased sed expression. However, one strain exhibited a trend towards increased sed expression under NaCl stress. No significant effect of agr on sed expression was detected and only one ΔsigB mutant showed a significant decrease in sed expression in the early stationary phase under NaCl stress. One ΔsarA mutant showed decreased sed expression in the early stationary and another increased sed expression in the stationary growth phase under NaCl stress. These findings suggest high strain-specific variation in sed expression and its regulation under NaCl stress.

The formation of Staphylococcus aureus enterotoxin in food environments and advances in risk assessment

The recent finding that the formation of staphylococcal enterotoxins in food is very different from that in cultures of pure Staphylococcus aureus sheds new light on, and brings into question, traditional microbial risk assessment methods based on planktonic liquid cultures. In fact, most bacteria in food appear to be associated with surfaces or tissues in various ways, and interaction with other bacteria through molecular signaling is prevalent. Nowadays it is well established that there are significant differences in the behavior of bacteria in the planktonic state and immobilized bacteria found in multicellular communities. Thus, in order to improve the production of high-quality, microbiologically safe food for human consumption, in situ data on enterotoxin formation in food environments are required to complement existing knowledge on the growth and survivability of S. aureus. This review focuses on enterotoxigenic S. aureus and describes recent findings related to enterotoxin formation in food environments, and ways in which risk assessment can take into account virulence behavior. An improved understanding of how environmental factors affect the expression of enterotoxins in foods will enable us to formulate new strategies for improved food safety.

Staphylococcus aureus virulence and metabolism are dramatically affected by Lactococcus lactis in cheese matrix

In complex environments such as cheeses, the lack of relevant information on the physiology and virulence expression of pathogenic bacteria and the impact of endogenous microbiota has hindered progress in risk assessment and control. Here, we investigated the behaviour of Staphylococcus aureus, a major foodborne pathogen, in a cheese matrix, either alone or in the presence of Lactococcus lactis, as a dominant species of cheese ecosystems. The dynamics of S. aureus was explored in situ by coupling a microbiological and, for the first time, a transcriptomic approach. Lactococcus lactis affected the carbohydrate and nitrogen metabolisms and the stress response of S. aureus by acidifying, proteolysing and decreasing the redox potential of the cheese matrix. Enterotoxin expression was positively or negatively modulated by both L. lactis and the cheese matrix itself, depending on the enterotoxin type. Among the main enterotoxins involved in staphylococcal food poisoning, sea expression was slightly favoured in the presence of L. lactis, whereas a strong repression of sec4 was observed in cheese matrix, even in the absence of L. lactis, and correlated with a reduced saeRS expression. Remarkably, the agr system was downregulated by the presence of L. lactis, in part because of the decrease in pH. This study highlights the intimate link between environment, metabolism and virulence, as illustrated by the influence of the cheese matrix context, including the presence of L. lactis, on two major virulence regulators, the agr system and saeRS.

Transcriptional variation of regulatory and virulence genes due to different media in Staphylococcus aureus

The pathogenesis of staphylococcal infections is a multifactorial process that depends on expression of different virulence factors. Expression of these factors is controlled by multiple regulatory systems in conjunction with environmental signals. Most of the genetic studies in Staphylococcus aureus have been performed using different growth media, therefore, we examined the effects of different growth media on transcription of the selective target (e.g., hla, hlb, spa, sspA) and regulatory (e.g., agr, sarA family) genes. The results from this study suggest that different growth media have substantial effect on transcription of various genes being analyzed. Interestingly, when compared with the wild-type, the isogenic sarA mutant showed a media-dependent distinct regulatory effect on expression of the target genes.

Influences of sigmaB and agr on expression of staphylococcal enterotoxin B (seb) in Staphylococcus aureus

In Staphylococcus aureus, enterotoxin B (SEB) is a superantigen that activates host interleukins and induces adverse responses, ranging from food poisoning to toxic shock. The alternate sigma factor, sigmaB (SigmaB), and agr are two known regulators of S. aureus. Northern blots of strain COL, a sigB-positive strain, showed an inverse correlation between sigmaB expression and seb message. seb expression was also measured as a function of a seb promoter linked to green fluorescent protein (GFP) expression in RN6390, COL, and Newman. In sigB mutants of RN6390, SH1000, COL, and Newman, seb promoter activities, as measured by GFP expression, increased relative to the respective parental types but at differing levels, suggesting alternate strain-specific regulation. In agr mutants of RN6390 and Newman, seb promoter activities were intermediate between the high level seen for the sigB mutant and the low level in the sigB active strains. A sigB agr double mutant of RN6390 displayed lower GFP expression than the agr mutant. These results suggest that while sigmaB and agr regulate seb expression in a divergent manner, other activator(s) of seb that depend on sigB expression may be present in S. aureus.

Bacterial virulence as a target for antimicrobial chemotherapy

As bacterial resistance to currently used antibiotics increases, so too must efforts to identify novel agents and strategies for the prevention and treatment of bacterial infection. In the past, antimicrobial drug discovery efforts have focused on eradicating infection by either cidal or static agents, resulting in clearance of the bacterium from the infected host. To this end, drug discovery targets have been those proteins or processes essential for bacterial cell viability. However, inhibition of the interaction between the bacterium and its host may also be a target. During establishment of an infection, pathogenic bacteria use carefully regulated pathways of conditional gene expression to transition from a free-living form to one that must adapt to the host milieu. This transition requires the regulated production of both extracellular and cell-surface molecules, often termed virulence factors. As the biological imperatives of the invading organism change during the course of an infection, the expression of these factors is altered in response to environmental cues. These may be changes in the host environment, for example, pH, metabolites, metal ions, osmolarity, and temperature. Alternatively, effector molecules produced by the bacterium to sense changing cell density can also lead to changes in virulence gene expression. Although the mechanisms of pathogenesis among different bacteria vary, the principles of virulence are generally conserved. Bacterial virulence may therefore offer unique opportunities to inhibit the establishment of infection or alter its course as a method of antimicrobial chemotherapy.

Effect of low water activity on staphylococcal enterotoxin A and B biosynthesis

Staphylococcus aureus strains FDA 743 (staphylococcal enterotoxin A [SEA]-producing), FDA 778 (staphylococcal enterotoxin B [SEB]-producing), and S6 (SEA- and SEB-producing) were used to examine the effect of low water activity (a(w)) on SEA and SEB biosynthesis. In this report, we show that SEB production is more sensitive to low a(w) than SEA production. We also show that when proline is available as a compatible solute for S. aureus, SEB production is significantly stimulated at low a(w). This stimulatory effect was not observed when other compatible solutes (i.e., glycine betaine or carnitine) were added to low a(w) growth media. Finally, Northern blot analysis revealed that the stimulation of SEB production at low a(w) by added proline occurs at the level of transcription.

The role of environmental factors in the regulation of virulence-determinant expression in Staphylococcus aureus 8325-4

Staphylococcus aureus is a major human pathogen, which produces a variety of virulence determinants. To study environmental regulation of virulence-determinant production, several transcriptional reporter gene fusions were constructed. Chromosomal fusions were made with the staphylococcal accessory regulator (sarA), alpha-haemolysin (hla), surface protein A (spa) and toxic-shock syndrome toxin-1 (tst) genes. The effect of many different environmental conditions on the expression of the fusions was examined. Expression of hla, tst and spa was strongly repressed in the presence of sodium chloride (1 M) or sucrose (20 mM), but sarA was relatively unaffected. The global regulator of expression of virulence-determinant genes, agr (accessory gene regulator) was not involved in the salt or sucrose repression. Novobiocin, a DNA gyrase inhibitor, did not significantly increase the expression of tst in wild-type or agr backgrounds and failed to relieve the salt suppression. Expression of tst was strongly stimulated in several low-metal environments, independently of agr, whilst spa levels were significantly reduced by EGTA. The complex, interactive role of environmental factors in the control of expression of the virulence determinants is discussed.

Cloning and nucleotide sequencing of a Staphylococcus aureus gene encoding a branched-chain-amino-acid transporter

We recently characterized a transposon-induced NaCl-sensitive mutant of Staphylococcus aureus (U. Vijaranakul, M. J. Nadakavukaren, D. O. Bayles, B. J. Wilkinson, and R. K. Jayaswal, Appl. Environ. Microbiol. 63:1889-1897, 1997). To further characterize this mutant, we determined the nucleotide sequence at the insertion site of the transposon on the S. aureus chromosome. Nucleotide sequencing revealed a 1,326-bp open reading frame (ORF442) encoding a hydrophobic 442-amino-acid polypeptide with a calculated molecular mass of 49,058 Da. The hydrophilicity profile of the gene product revealed the existence of 12 hydrophobic domains predicted to form membrane-associated alpha-helices. Comparison of the amino acid sequence of ORF442 with amino acid sequences in the GenBank database showed extensive homology with the branched-chain-amino-acid transport genes of gram-positive and gram-negative bacteria. This is the first brnQ gene in staphylococci to be described.

Role of the accessory gene regulator (agr) in pathogenesis of staphylococcal osteomyelitis

To examine the role of the accessory gene regulator (agr) in staphylococcal osteomyelitis, we compared a Staphylococcus aureus osteomyelitis isolate (UAMS-1) with a derivative of the same strain (UAMS-4) carrying an inactivated agr locus. Virulence was assessed with a rabbit model of acute, exogenous osteomyelitis. Bacteria were delivered by microinjection into the midradial region of the forelimb. After 4 weeks, UAMS-1 was identified in the bone of 12 of 13 rabbits infected with > or = 2 x 10(6) CFU and 5 of 6 infected with < or = 2 x 10(5) CFU. In contrast, UAMS-4 was found in 6 of 13 infected with the higher dose and 1 of 6 infected with the lower dose. Additionally, on the basis of a five-point scale assessing radiographic evidence of disease, rabbits infected with UAMS-1 had average scores of 2.64 +/- 0.30 (high dose) and 1.43 +/- 0.39 (low dose) while rabbits infected with UAMS-4 had average scores of 0.95 +/- 0.23 (high dose) and 0.63 +/- 0.20 (low dose). Uninfected controls had an average score of 0.53 +/- 0.08. The results obtained with UAMS-1 were significantly different from those obtained with UAMS-4 at both doses (P < or = 0.047). The results obtained with UAMS-4 were not significantly different from those obtained with the controls at either dose of UAMS-4 (P > or = 0.150). On the basis of a similar five-point scale assessing histopathological evidence of disease, rabbits infected with UAMS-1 had average scores of 2.31 +/- 0.22 (high dose) and 1.96 +/- 0.36 (low dose) while rabbits infected with UAMS-4 had average scores of 1.58 +/- 0.29 (high dose) and 0.83 +/- 0.32 (low dose). Controls had an average score of 0.33 +/- 0.05. The results obtained with UAMS-1 were significantly different from those obtained with UAMS-4 at both doses (P < or = 0.040). However, the results obtained with UAMS-4 were significantly different from the controls only at the high dose of UAMS-4 (P = 0.025). We conclude that mutation of agr reduces the incidence and severity of disease but does not eliminate the ability to colonize bone and cause histopathological evidence of osteomyelitis.

The extracellular protein regulator (xpr) affects exoprotein and agr mRNA levels in Staphylococcus aureus

xpr, a regulatory element of exoprotein synthesis in Staphylococcus aureus, defined by an insertion of Tn551 into the chromosome of strain S6C, affects the expression of several exoproteins at the mRNA level. Drastic reduction in transcript levels for staphylococcal enterotoxin B (seb), lipase (geh), alpha-toxin (hla), and delta-toxin (hld) were detected, while mRNA levels for coagulase (coa) and protein A (spa) were elevated. Because the delta-toxin gene resides within the RNAIII transcript of the exoprotein regulator, agr, the reduction in hld message in the mutant strain of S6C is indicative of additional regulatory events in exoprotein gene expression. Northern (RNA) analysis of total cellular RNA hybridized with probes specific for RNAII and RNAIII (the two major transcripts of the agr operon) showed that both transcripts were reduced 16- to 32-fold at 3 h (late exponential phase) and 8- to 16-fold at 12 h (postexponential phase). These data confirm our original findings (M. S. Smeltzer, M. E. Hart, and J. J. Iandolo, Infect. Immun. 61:919-925, 1993) that two regulatory loci, agr and xpr, are interactive at the genotypic level.

Cloning, expression, and mutagenesis of phosphatidylinositol-specific phospholipase C from Staphylococcus aureus: a potential staphylococcal virulence factor

Staphylococcus aureus secretes a phosphatidylinositol (PI)-specific phospholipase C (PI-PLC) which is able to hydrolyze the membrane lipid PI and membrane protein anchors containing glycosyl-PI. The gene for PI-PLC (plc) was cloned from S. aureus into Escherichia coli. Oligonucleotide probes based on partial protein sequence and polyclonal antibodies raised against the purified protein were used to identify positive clones. E. coli transformed with a plasmid containing the plc gene expressed PI-PLC enzyme activity which was abolished by mutagenesis with a tetracycline resistance gene. The plc gene was present in all 15 S. aureus strains examined but not in any of 6 coagulase-negative staphylococcal species. The plc gene contained 984 bp and coded for a mature protein with a calculated molecular mass of 34,107 Da. Amino acid sequence comparisons indicated that the staphylococcal plc gene was similar (51 to 56%) to the PI-PLCs from Bacillus cereus, Bacillus thuringiensis, and Listeria monocytogenes. The recombinant PI-PLC expressed in E. coli was purified and exhibited biochemical properties identical to those of the native PI-PLC from S. aureus. PI-PLC production was decreased in agr mutant strains of S. aureus. However, PI-PLC production by both agr+ and agr mutant strains exhibited a similar dependence on the type of medium used. These data suggested that PI-PLC production was regulated by both agr-dependent and agr-independent mechanisms.