Urinary zinc excretion in a patient with burns: a caveat when using bladder catheters in urine zinc studies

Antimicrobial Resistance: Two-Component Regulatory Systems and Multidrug Efflux Pumps

The number of multidrug-resistant bacteria is rapidly spreading worldwide. Among the various mechanisms determining resistance to antimicrobial agents, multidrug efflux pumps play a noteworthy role because they export extraneous and noxious substrates from the inside to the outside environment of the bacterial cell contributing to multidrug resistance (MDR) and, consequently, to the failure of anti-infective therapies. The expression of multidrug efflux pumps can be under the control of transcriptional regulators and two-component systems (TCS). TCS are a major mechanism by which microorganisms sense and reply to external and/or intramembrane stimuli by coordinating the expression of genes involved not only in pathogenic pathways but also in antibiotic resistance. In this review, we describe the influence of TCS on multidrug efflux pump expression and activity in some Gram-negative and Gram-positive bacteria. Taking into account the strict correlation between TCS and multidrug efflux pumps, the development of drugs targeting TCS, alone or together with already discovered efflux pump inhibitors, may represent a beneficial strategy to contribute to the fight against growing antibiotic resistance.

CzcR-CzcS, a Two-component System Involved in Heavy Metal and Carbapenem Resistance in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an environmental bacterium involved in mineralization of organic matter. It is also an opportunistic pathogen able to cause serious infections in immunocompromised hosts. As such, it is exposed to xenobiotics including solvents, heavy metals, and antimicrobials. We studied the response of P. aeruginosa upon exposure to heavy metals or antibiotics to investigate whether common regulatory mechanisms govern resistance to both types of compounds. We showed that sublethal zinc concentrations induced resistance to zinc, cadmium, and cobalt, while lethal zinc concentrations selected mutants constitutively resistant to these heavy metals. Both zinc-induced and stable zinc-resistant strains were also resistant to the carbapenem antibiotic imipenem. On the other hand, only 20% of clones selected on imipenem were also resistant to zinc. Heavy metal resistance in the mutants could be correlated by quantitative real time PCR with increased expression of the heavy metal efflux pump CzcCBA and its cognate two-component regulator genes czcR-czcS. Western blot analysis revealed reduced expression of the basic amino acid and carbapenem-specific OprD porin in all imipenem-resistant mutants. Sequencing of the czcR-czcS DNA region in eight independent zinc- and imipenem-resistant mutants revealed the presence of the same V194L mutation in the CzcS sensor protein. Overexpression in a susceptible wild type strain of the mutated CzsS protein, but not of the wild type form, resulted in decreased oprD and increased czcC expression. We further show that zinc is released from latex urinary catheters into urine in amounts sufficient to induce carbapenem resistance in P. aeruginosa, possibly compromising treatment of urinary tract infections by this class of antibiotics.