Chance Favors the Prepared Bacterium

Excerpted from a backgrounder on extended-spectrum beta-lactamase enzymes written in 2002.

An attempt to stay one step ahead of Beta-lactamase-secreting bacteria prompted a frenzy of antibiotic invention of during the 1970s and 1980s and yielded an impressive array of new agents, including an important group of beta-lactam drugs, the oxyimino-cephalosporins. These "expanded-spectrum" antibacterials quickly became a favorite treatment for patients with, or even at risk of, serious gram-negative infections.

Bacteria outmaneuver scientists
Calling on billions of years of practical experience, bacteria quickly reconfigured their existing beta-lactamase enzymes to defeat the new antibiotics. Because the altered enzymes had extended activity, notably against the oxyimino-cephalosporins, they were designated "extended-spectrum beta-lactamases" or, for short: "ESBLs." Organisms armed with these particular weapons were in luck whenever oxyimino-cephalosporins were administered (which they were in abundance); surviving the treatment to multiply rapidly, filling the void left by their unprepared bacterial counterparts.

A Gram-Negative Double Whammy

ESBLs are most often identified with the ubiquitous Gram-negative bacteria Klebsiella pneumoniae, Escherichia coli and Pseudomonas aeruginosa, endowing these organisms with high levels of resistance to ceftazidime and other expanded-spectrum cephalosporins as well as to the penicillin, aztreonam.

Beta-lactamases are stored in the periplasmic space of Gram-negative bacteria such as K. pneumoniae, E. coli and P. aeruginosa which also have selective porins, restricting antibiotic entry into the cell. Less drug gets into these cells and the drug that does get in is hydrolyzed by internal beta-lactamases. Thus gram-negative microbes can achieve high levels of resistance with lower levels of enzyme than their gram-positive counterparts --which, like the multi-talented Staphylococcus aureus, also produce beta-lactamases but excrete the enzymes rather than store them.*

Resistance is easily transferred from one bacterium to another
The genes responsible for extended-spectrum beta-lactamases are readily transferred from one bacterium to another via plasmids. Plasmids are typically circular DNA molecules, quite distinct from the organism's cellular genetic material, and highly mobile. Interestingly, some plasmid-bearing beta-lactamase DNA originated from chromosomal genes of various Enterobacter spp., Citrobacter freundii or P. aeruginosa. Resistance genes for aminoglycosides, chloramphenicol, tetracyclines, and sulfonamides are also incorporated into these particular plasmids.

ESBL detection isn't very reliable
Which is unfortunate because accurate detection of ESBLs is important. Firstly because there?s an increasing prevalence of enzyme-producing bacteria worldwide. Secondly, an "inoculum effect" is often associated with infections caused by ESBL-producing bacteria [more organisms --> more enzyme = greater levels of resistance]. Clinically, there are a great many diseases, pneumonia for example, where the bacterial load can be very great.

Clinical microbiology: Although most ESBLs confer resistance to one or more of the oxyimino-beta-lactam drugs, the minimal inhibitory capacity (MIC) isn't increased enough to be deemed resistant by the National Committee for Clinical Laboratory Standards (NCCLS) guidelines. In other words, the guidelines don't predict bacterial susceptibility accurately.

Molecular detection: DNA probes are labor intensive and can?t distinguish between ESBLs and non-ESBLS. PCR, ditto although easier to do. Oligotyping, labor intensive and can't detect new variants. Ad nauseum. (See Table 6, p. 943 in Bradford 1 for more information.)

ESBLs make treatment difficult
Experts agree that patients infected with ESBL-producing bacteria are at increased risk for treatment failure and these enzymes now present a problem for hospitalized patients worldwide. In the U.S., ESBL production in Enterobacteriaceae ranges from 0 to 25% depending on the institution (See CDC National Nosocomial Infections Surveillance).

The Sickest Patients are at Greatest Risk

A common theme among hospitals plagued by ESBL-producing pathogens is a high volume and indiscriminate use of expanded-spectrum cephalosporins. Specific risk factors include lengthy hospital stays, severe illness, extended time in the ICU, intubation and mechanical ventilation, urinary or arterial cauterization, and whether or not the patient has had prior antibiotic treatment.

In addition to infections acquired in ICUs, surgical wards and the hospital in general, ESBL-producing bacteria are also being isolated with increasing frequency from patients in extended-care facilities. Recent outbreaks have been caused by organisms carrying multiple beta lactamases. Many of the isolated pathogens proved resistant to beta-lactamase-beta-lactamase inhibitor combinations, cephamycins and carbapenems in addition to the oxyimino-cephalosporins and aztreonam.

Anecdotal reports proved wrong
While there have been anecdotal reports of good clinical outcomes with cephalosporins despite infection with ESBL-producing bacteria, better controlled investigations have shown otherwise.

For example, one research team reports that 54% of patients with serious infections caused by lab-documented "susceptible" ESBL-producing bacteria failed on therapy. All the patients whose therapy failed either died or had to have their medication changed. They caution that: "Clinical microbiology laboratories should take heed of current recommendations for detection of ESBLs in order to avoid potential treatment failure when cephalosporins are used."

In another study researchers described treatment experience with extended-spectrum cephalosporins for patients with bloodstream infections caused by ESBL-producing E. coli (23 episodes) and K. pneumoniae (13 episodes). Ceftazidime treatment was associated with therapeutic failure in all patients. The authors: "...strongly recommend that the use of ceftazidime for empirical therapy should be avoided whenever possible."

However, there are also published reports that two cephalosporins appearing very active in the laboratory may prove less-than-reliable in the clinics. For example, cefepime and cefpirome have greater intrinsic potency than ceftazidime because they're more streamlined and penetrate a bacterium with greater ease. However, when enough ESBL-producing E. coli and K. pneumoniae are present the inoculum effect may negate any therapeutic advantage provided by these antibiotics.

Antibiotic Cycling

Antibiotic cycling is based on the notion that limiting general exposure to an antibiotic will reduce the likelihood of resistance. According to one infectious disease physician, if antibiotics with high resistance potential are used, the strategy could backfire. According to Burke Cunha the belief that antibiotic resistance is related to certain antibiotic classes --third-generation cephalosporins or fluoroquinolones--is a "widespread myth." He says that some antibiotics have high-resistance potential (ceftazidime, ciprofloxacin and imipenem) and should be eliminated at the formulary level. He points out that antibiotics such as vancomycin, doxycycline and nitrofurantoin have been used in high volumes for decades without bacteria becoming resistant to them. (Confronted with Enterococcus resistance to vancomycin, Cunha says that that the degree of E. faecalis resistance hasn't increased, rather the prevalence of another organism, E. faecium has. Enterococcus faecium is the vancomycin-resistant enterococcus.)

Thus, if antibiotic cycling is to be effective, high-resistance potential antibiotics should be replaced with agents having low resistance potential. In fact, he suggests formulary deletion of resistance-prone antibiotics everywhere and forever.

Summary

Beta-lactamases are the favorite defensive weapon of most if not all clinically important gram-negative pathogens. When confronted with new beta lactam antibiotics these organisms quickly redesign the enzymes they have and/or borrow the genes for new ones. Resistance spreads rapidly as beta lactamase genes are passed from one bacterium to another with ease via plasmids. When an inappropriate antibiotic is used chance favors ESBL-protected cells. They're selected out, multiply unopposed and eventually replace susceptible microbes to become the predominant bacteria.

ESBL production correlates with clinical failure. A number of well-controlled studies document poor outcomes when patients infected with ESBL-producing organisms were treated with easily hydrolyzed antibiotics, especially if they are seriously ill. Unfortunately, in general (the real world) ESBL testing is not well-standardized and largely inaccurate. Thus it is recommended that resistance-prone agents be retired from the hospital formulary.

References include:
Bradford PA. Extended-spectrum Beta lactamases in the 21st century: characterization, epidemiology, and detection of this important resistance threat. Clinical Microbiology Reviews 2001; 14:993-951.

Karchmer AW. Cephalosporins. In: Mandell G.L., Bennett J.E., Dolin R, editors. Principles and Practice of Infectious Disease. Philadelphia: Churchilll Livingstone, 2000: 274-291.

Salyers A, Whitt DD. Bacterial Pathogenesis. A Molecular Approach. 2 ed. Washington, D.C: ASM Press, 2002.

Thomson KS, Moland ES. Cefepime, Piperacillin-Tazobactam, and the Inoculum Effect in Tests with Extended-Spectrum Beta-Lactamase-Producing Enterobacteriaceae. Antimicrob Agents Chemother 2001; 45(12):3548-3554.

Paterson DL, Ko W, Von Gottengerg A, et al. Outcome of cephalosporin treatment for serious infections due to apparently susceptible organisms producing extended-spectrum beta-lactamases: Implications for the clinical microbiology laboratory. Journal of Clinical Microbiology 2001; 39(6):2206-2212.

Wong-Beringer A, Hindler J, Loeloff M, et al. Molecular correlation for the treatment outcomes in bloodstream infections caused by Escherichia coli and Klebsiella pneumoniae with reduced susceptibility to ceftazidime. Clinical Infectious Diseases 2002; 34(2):135-146.

Cunha BA. Effective antibiotic-resistance control strategies (Commentary). The Lancet 2001; 357:1-3.

Footnote

*Note that the beta-lactamases excreted by Gram-positive bacteria protect all organisms in the immediate environment as does enzyme liberated from dead bacteria.