NorB, an Efflux Pump in Staphylococcus aureus Strain MW2, Contributes to Bacterial Fitness in Abscesses

Abstract

While remaining a major problem in hospitals, Staphylococcus aureus is now spreading in communities. Strain MW2 (USA400 lineage) and other community methicillin-resistant S. aureus strains most commonly cause skin infections with abscess formation. Multidrug resistance (MDR) efflux pumps contribute to antimicrobial resistance but may also contribute to bacterial survival by removal of environmental toxins. In S. aureus, NorA, NorB, NorC, and Tet38 are chromosomally encoded efflux pumps whose overexpression can confer MDR to quinolones and other compounds (Nor pumps) or tetracyclines alone (Tet38), but the natural substrates of these pumps are not known. To determine the role of these efflux pumps in a natural environment in the absence of antibiotics, we used strain MW2 in a mouse subcutaneous abscess model and compared pump gene expression as determined by reverse transcription-PCR in the abscesses and in vitro. norB and tet38 were selectively upregulated in vivo more than 171-and 24-fold, respectively, whereas norA and norC were downregulated. These changes were associated with an increase in expression of mgrA, which encodes a transcriptional regulator known to affect pump gene expression. In competition experiments using equal inocula of a norB or tet38 mutant and parent strain MW2, each mutant exhibited growth defects of about two-to threefold in vivo. In complementation experiments, a single-copy insertion of norB (but not a single-copy insertion of tet38) in the attB site within geh restored the growth fitness of the norB mutant in vivo. Our findings indicate that some MDR pumps, like NorB, can facilitate bacterial survival when they are overexpressed in a staphylococcal abscess and may contribute to the relative resistance of abscesses to antimicrobial therapy, thus linking bacterial fitness and resistance in vivo.

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

“…Perhaps more likely is the possibility that low pH serves as a signal to the bacterial cell of its presence in an environment in which increased expression of NorB is beneficial for other reasons. One such notable example is a bacterial abscess in which low pH may be a signal for the increased expression of norB that we have shown occurs in this environment (8). Although its natural substrate in an abscess environment remains unknown, NorB has been shown to contribute selectively to bacterial survival in this common type of S. aureus infection.…”

“…Increased expression of norB, encoding the NorB efflux pump, causes reduced susceptibility to moxifloxacin in vitro and contributes to bacterial fitness in a subcutaneous abscess model (8). Since norB expression was increased shortly after a shift to pH 4.5, we wished to determine whether susceptibility to moxifloxacin, a NorB substrate (24), was also affected by a shift to pH 4.5.…”

“…In contrast, under the same pH conditions, other genes encoding drug efflux pumps, such as emrB/qacA and norC, exhibit substantial increases (12-fold to 15-fold, respectively) in transcript levels (2,5). Our previous studies showed that the MDR efflux pump NorB, which confers resistance to certain quinolones, such as moxifloxacin, contributes to bacterial survival when it is selectively overexpressed in subcutaneous abscesses (8). Since the pH in the abscess environment can be as low as 5.5 or 6, it is possible that this signal may be responsible in part for the changes in norB expression seen in abscesses.…”

Mechanisms of NDV-3 vaccine efficacy in MRSA skin versus invasive infection

“…Balb/C mice (Harlan) were immunized as described above. An s.c. skin/skin structure abscess model was implemented for these studies as modified from Voyich et al (84) and Ding et al (85). In brief, on study day 35, mice were anesthetized, their flanks were shaved and sterilized, and the mice were randomized into appropriate study groups.…”

Evaluation of Different Drugs in Down-Regulation of Efflux Pump Genes expression in Methicillin-Resistant <I>Staphylococcus aureus</I> Strains

“…Several studies have shown that increased resistance to antibiotics, biocides and dyes has been associated with NorA and NorB mediated efflux pumps (Handzlik et al, 2013;Sabatini et al, 2012;Truong-Bolduc et al, 2005;Ding et al, 2008). However, antimicrobial resistance associated with NorA and NorB efflux pumps is poorly characterized in S. aureus (Handzlik et al, 2013).…”