Effect of Mild Acid on Gene Expression in Staphylococcus aureus

Abstract

During staphylococcal growth in glucose-supplemented medium, the pH of a culture starting near neutrality typically decreases by about 2 units due to the fermentation of glucose. Many species can comfortably tolerate the resulting mildly acidic conditions (pH, ϳ5.5) by mounting a cellular response, which serves to defend the intracellular pH and, in principle, to modify gene expression for optimal performance in a mildly acidic infection site. In this report, we show that changes in staphylococcal gene expression formerly thought to represent a glucose effect are largely the result of declining pH. We examine the cellular response to mild acid by microarray analysis and define the affected gene set as the mild acid stimulon. Many of the genes encoding extracellular virulence factors are affected, as are genes involved in regulation of virulence factor gene expression, transport of sugars and peptides, intermediary metabolism, and pH homeostasis. Key results are verified by gene fusion and Northern blot hybridization analyses. The results point to, but do not define, possible regulatory pathways by which the organism senses and responds to a pH stimulus.Facultative pathogens such as staphylococci produce a wide variety of accessory proteins. Many of these are involved in, or required for, infectivity and are collectively referred to as the virulon. It is widely believed that the genes encoding these proteins are regulated according to the exigencies of the local sites in which the organism is able to establish an infection. The evidence in support of this very logical view is, at best, rather sketchy, largely for two reasons. Firstly, it has, until very recently, been unapproachably difficult to determine the putative locale-specific gene expression patterns. Secondly, it has been no easy matter to identify the specific environmental factors that would determine these patterns. As a preamble to the ultimate objective of defining in vivo environmental factors that influence the expression of accessory genes involved in local pathogenesis, we have begun to reexamine the effect of certain nutrients and other chemicals, including glucose, acidic pH, salt, salicylic acid, and subinhibitory concentrations of antibiotics, on the expression of virulence genes during growth in vitro. These substances have widespread effects on the transcription of virulence genes and other accessory genes, which are presumably initiated through signaling elements and mediated through the complex regulatory network that governs the expression of all accessory genes (34). Our entrée into the transduction of environmental signals was provided by the observation that all of these substances affected transcription of a key signaling locus, sae, which is involved in the regulation of the expression of many exoproteins (36). In that study, which was initiated by a determination of the effect of glucose, it was shown that sae has a complex transcriptional pattern that undergoes a very striking switch during in vitro growth at neutral pH, in which...

Contrasting Effects of Acidic pH on the Extracellular and Intracellular Activities of the Anti-Gram-Positive Fluoroquinolones Moxifloxacin and Delafloxacin againstStaphylococcus aureus

“…This property further increases delafloxacin's potency in acidic environments (35,43), as also observed for N-sulfanilylpiperazinyl fluoroquinolones (1,3) and finafloxacin (21). This could be an advantage with respect to S. aureus infections, since this bacterium, which shows a high tolerance to low pH, survives and multiplies in mild acidic environments (47), such as the skin, the vagina, or the urinary tract, and within the phagolysosomes of infected cells (6), where the pH is about 5 to 5.5 (19). Thus, delafloxacin contrasts with what is observed for many of the current antistaphylococcal antibiotics for which activity is reduced when the pH is lowered, as seen for other fluoroquinolones (e.g., norfloxa-cin, ciprofloxacin, or moxifloxacin [6,48]), macrolides (26), clindamycin (37), and gentamicin (7).…”

Implication of the NorB Efflux Pump in the Adaptation of Staphylococcus aureus to Growth at Acid pH and in Resistance to Moxifloxacin

“…These adaptations are multifaceted, involving changes in expression of many genes with various functions, but the specific mechanisms are not well studied. The acid stress response has been shown to include increased transcript levels of genes involved in utilization of urease (ureA, ureB, ureC, and ureF) and of nuoF, which encodes an NADH dehydrogenase, which can increase internal cell pH by assisting proton export from the bacterial cell (1,2,5,6,28). A decrease in transcripts encoding the F 0 F 1 ATPase was also observed, which was somewhat surprising since this F 0 F 1 ATPase is considered an important proton extrusion mechanism in several Gram-positive bacterial species, such as Streptococcus mutans (17).…”

“…S. aureus is also carried by healthy humans as part of the normal flora of the nose, throat, and skin. This pathogen can adapt to various unfavorable environmental conditions or stresses caused by variation in pH, temperature, or nutritive sources by activating several defense mechanisms, including the use of proton pumps to maintain the cellular pH at an acceptable level, or shifting the growth energy to adaptation mechanisms (1,2,28). These conditions may also serve as signals to the organism that are important for pathogenesis and fitness at specific sites of infection or colonization of the host.…”

“…pH is known to have broad effects on gene expression in S. aureus (2,28), but little is known about the role of bacterial efflux transporters in response to pH. Expression of genes encoding certain multidrug resistance (MDR) transporters, such as norA, encoding the NorA efflux pump, and bcr, encoding the bicyclomycin resistance phenotype, are downregulated under low-pH conditions.…”

Disruption of the alsSD operon of Enterococcus faecalis impairs growth on pyruvate at low pH

“…An acidic induction of C4 genes has also been described for Klebsiella terrigena (Mayer et al, 1995), Staphylococcus aureus (Weinrick et al, 2004) and B. subtilis (Wilks et al, 2009). These data prompted us to analyse whether the Pals promoter of Ent.…”