How much do you know about bacterial resistance? Today our story isn’t about architecture, but about something much smaller. We’re standing in front of what you see as the AMPC. But what I’m thinking about are tiny organisms and their incredible ability to adapt.
The AMPC may appear like any other building. Yet, its name echoes a battle being waged on a microscopic scale. AmpC beta-lactamases are enzymes produced by bacteria that cause resistance to antibiotics. Think of it as an arms race. Scientists are constantly developing new drugs. Bacteria constantly evolve defenses.
This isn’t some obscure scientific concept. It directly impacts human health. Infections once easily treated are becoming increasingly difficult to combat. The AMPC beta-lactamases are clinically important cephalosporinases. They’re encoded on the chromosomes of many Enterobacteriaceae. They mediate resistance to cephalothin, cefazolin, cefoxitin, most penicillins, and beta-lactamase inhibitor-beta-lactam combinations. In many bacteria, AmpC enzymes are inducible and can be expressed at high levels by mutation. Overexpression confers resistance to broad-spectrum cephalosporins. These include cefotaxime, ceftazidime, and ceftriaxone. This is a problem in infections due to Enterobacter aerogenes and Enterobacter cloacae. An isolate initially susceptible to these agents may become resistant upon therapy.
These enzymes were first identified in E. coli back in 1940. Mutations were observed that stepwise enhanced resistance. The actual sequence of the ampC gene from E. coli was only reported in 1981. Researchers discovered that the AmpC enzymes belong to class C.
What makes things even more complicated is that genes for AmpC enzymes can be acquired by transmissible plasmids. Now, bacteria lacking or poorly expressing a chromosomal blaAmpC gene can show resistance. Think Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis.
These enzymes, whether from chromosomes or plasmids, are also evolving to hydrolyze broad-spectrum cephalosporins more efficiently. The conservative listing of AmpC beta-lactamases includes proteins with the structure from organisms that have been demonstrated to possess appropriate AmpC-type beta-lactamase activity. The phylum Proteobacteria contains the largest number, but at least one acid-fast actinobacterium also produces AmpC beta-lactamase. The clinical microbiologist needs to consider this resistance mechanism whatever the identification of an organism.
It’s a complex system. In many Enterobacteriaceae, AmpC expression is low but inducible in response to beta-lactam exposure. The most common cause of AmpC overexpression in clinical isolates is a mutation in ampD leading to AmpC hyperinducibility or constitutive hyperproduction.
Carbapenems can usually be used to treat infections due to AmpC-producing bacteria. However, carbapenem resistance can arise in some organisms. This can happen by mutations that reduce influx or enhance efflux.
So as we admire the AMPC, remember it’s a name that represents the constant struggle. It highlights bacteria’s ability to adapt. It is a reminder of the importance of responsible antibiotic use and continued scientific research. It’s about staying one step ahead in this microscopic arms race.