1: Antimicrob. Agents Chemother. 2007 May;51(5):1627-32. Epub 2007 Feb 16.

Role of Acidic pH in the Susceptibility of Intraphagocytic Methicillin-Resistant
Staphylococcus aureus Strains to Meropenem and Cloxacillin.

Lemaire S, Van Bambeke F, Mingeot-Leclercq MP, Glupczynski Y, Tulkens PM.

Unité de Pharmacologie Cellulaire et Moléculaire, Université Catholique de
Louvain, UCL 73.70 Avenue E. Mounier 73, B-1200 Brussels, Belgium.

Early studies showed that methicillin-resistant Staphylococcus aureus (MRSA)
strains are susceptible to beta-lactams when they are exposed to pH </= 5.5 in
broth. Because S. aureus survives in the phagolysosomes of macrophages, where the
pH may be acidic, we have examined the susceptibility of MRSA ATCC 33591
phagocytized by human THP-1 macrophages to meropenem (MEM) and cloxacillin (CLX).
Using a pharmacodynamic model assessing key pharmacological (50% effective
concentration and maximal efficacy) and microbiological (static concentration)
descriptors of antibiotic activity, we show that intraphagocytic MRSA strains are
as sensitive to MEM and CLX as methicillin-susceptible S. aureus (MSSA; ATCC
25923). This observation was replicated in broth if the pH was brought to 5.5 and
was confirmed with clinical strains. Electron microscopy showed that both the
MRSA and the MSSA strains localized and multiplied in membrane-bounded structures
(phagolysosomes) in the absence of beta-lactams. Incubation of the infected
macrophages with ammonium chloride (to raise the phagolysosomal pH) made MRSA
insensitive to MEM and CLX. No difference was seen in mec, mecA, mecI, mecR1,
femA, and femB expression (reversed transcription-PCR) or in PBP 2a content
(immunodetection) in MRSA grown in broth at pH 5.5 compared with that in MRSA
grown in broth at 7.4. The level of [(14)C]benzylpenicillin binding to cell walls
prepared from a non-beta-lactamase-producing MRSA clinical isolate was two times
lower than that to cell walls prepared from MSSA ATCC 25923 at pH 7.4, but the
levels increased to similar values for both strains at pH 5.5. These data suggest
that the restoration of susceptibility of intraphagocytic of MRSA to MEM and CLX
is due to the acidic pH prevailing in phagolysosomes and is mediated by an
enhanced binding to penicillin-binding proteins.

PMID: 17307986 [Pubmed - in process]

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