Gram-negative Resistance


Gram-negative bacilli can develop resistance to most antibiotics through four general methods: production of enzymes that destroy the integrity of the antibiotic; mutations at the binding site, thereby preventing some antibiotics from binding tightly; downregulation of outer membrane proteins, thus preventing the antibiotic from getting into the periplasmic space; and efflux pumps that efficiently remove an antibiotic from the cell. Often for multidrug resistance to occur, several mechanisms of resistance are contained in the same strain.

The production of beta-lactamases is among the most common and clinically significant resistance mechanisms displayed by all gram-negative bacilli. beta-Lactamases are enzymes that hydrolyze the betalactam chemical structure and inactivate the drug. They are typically classified by either the Ambler classification or the Bush-Jacoby-Medeiros classification. With the Ambler classification, beta-lactamases are divided into four classes based on amino acid similarities. For example, classes A, C, and D are serine beta-lactamases, whereas class B enzymes are zinc beta-lactamases. The Bush-Jacoby-Medeiros classification divides beta-lactamases into four groups and multiple subgroups, based on substrate and inhibitor profiles. Specific types of beta-lactamases produced by multidrug-resistant gram-negative bacilli will be discussed later.

Gram-negative bacilli can also produce enzymes that destroy aminoglycosides (i.e., aminoglycoside-modifying enzymes); these can be acetyltransferases, adenyltransferases, or phosphoryltransferases.These enzymes chemically modify the aminoglycoside structure, thereby interfering with drug transport or preventing the antibiotic from binding to the 30S ribosomal subunit. Aminoglycoside-modifying enzymes are usually not solely responsible for aminoglycoside resistance in gram-negative bacilli, as impermeability, efflux, and novel aminoglycoside resistance gene cassettes are also to blame.

Other ways in which bacteria possess resistance is through reduced permeability and efflux pumps. Before binding to their target site, antibiotics must permeate through the outer membrane of the organisms through an outer membrane protein or porin. For example, loss of the 54-kD outer membrane protein, known as OprD, is the most common mechanism of imipenem resistance in P. aeruginosa. Often, reduced permeability works in conjunction with efflux pumps to reduce antibiotic concentrations and prevent accumulation in the bacteria. In P. aeruginosa, efflux pumps are most notably a three-component protein system located on the cytoplasmic membrane and the outer membrane porin. Resistance as a result of efflux pumps and/or porin changes is commonly associated with this pathogen. Similar to the beta-lactamases, other gram-negative bacilli are also in possession of these resistance mechanisms but vary in the characteristics of the component protein system.

Finally, binding-site mutations are the most common resistance mechanisms for gram-negative bacilli against the fluoroquinolones. Along with efflux, mutations of the target enzymes topoisomerase II and IV result in significant increases in fluoroquinolone minimum inhibitory concentrations (MICs). These enzymes are encoded by gyrA and parC, respectively, and among gram-negative organisms, it appears that mutations in gyrA are the most important, occurring before parC mutations.


ESBLS and Carbapenemase Resistance

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Adelaide – AIMS National Conference 2009

By Jan Bell

Modified Hodge Test


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