Acronymous Organisms: ESCAPPM and HACEK
Or is "acronymic" the correct adjective? In any case, the college loves to repeat this question. Recent iterations have included Question 3.3 from the first paper of 2014, Question 25.2 from the first paper of 2009 and Question 25.2 from the second paper of 2008.
The ESCAPPM organisms with induceable cephalosporinase
The following is a list of Gram-negative organisms who might occasionally appear to be sensitive to β-lactam antibiotics, but who in fact develop a resistance rapidly, owing to the overexpression of induceable chromosomal AmpC cephalosporinase/β-lactamase enzymes.
- Acinetobacter (and Aeromonas)
The linked article on this topic also identifies Chromobacterium violaceum, Enterobacter, E. coli, Hafnia alvei, Lysobacter lactamgenus, Ochrobactrum anthropi, Proteus rettgeri, Pseudomonas aeruginosa, Psychrobacter immobilis, Rhodobacter sphaeroides and Yersinia enterocolitica as carriers of the AmpC cephalosporinase/β-lactamase.
An intelligent person would not be satisfied with a mere list, and would go on to ask the question,
What the hell is this AmpC β-lactamase?
George A. Jacoby puts it best in his excellent article, which is the primary resource for this topic:
"AmpC β-lactamases are clinically important cephalosporinases encoded on the chromosomes of many of the Enterobacteriaceae and a few other organisms, where they mediate resistance to cephalothin, cefazolin, cefoxitin, most penicillins, and β-lactamase inhibitor-β-lactam combinations"
- AmpC β-lactamase is a chromosomal-mediated enzyme (unlike ESBL β-lactamase which is non-induceable and plasmid-mediated)
- In a given population of an ESCAPPM organism, there are mutants which express this β-lactamase. Within a short time during the course of treatment, this clone proliferates.
- AmpC β-lactamase is not inhibited by clavulanic acid, but some may be inhibited by tazobactam (thus there is a variable sensitivity to Tazocin).
The ESKAPE group
In addition to the ESCAPPM bunch, in 2009 the Infectious Diseases Society of America launched another acronym to describe the group of pathogens which - over the coming decades - are going to pose the greatest threat to mankind by virtue of their antimicrobial resistance:
- Enterococcus faecium
- Staphylococcus aureus
- Klebsiella pneumoniae
- Acinetobacter baumanii
- Pseudomonas aeruginosa
- Enterobacter sp.
Panic regarding the "post-antibiotic era" is spread further by anxious reports of such nightmarish creatures as a pan-drug-resistant Acinetobacter, which apparently reproduces quite comfortably in a pot of boiling meropenem.
The HACEK group of endocarditis organisms
These are a bunch of oropharyngeal Gram-negatives which had previously been thought to be frequently responsible for infective endocarditis, but are now known to be fairly rare, responsible for only about 3% of native valve endocarditis.
- Haemophilus species: H.aphrophilus, H.parainfluenzae and H.paraphrophilus
- Actinobacillus and Aggregatobacter species
- Cardiobacterium hominis
- Eikenella corrodens
- Kingella kingae
Why care about them, if they are such rare pathogens? Well. The slow growth of these organisms means the cultures are usually negative. Vegetations will be visible, and PCR testing will reveal the relevant pathogen, but the cultures will remain negative. Fortunately, most of them are sensitive to bog-standard ceftriaxone.