JAC Advance Access originally published online on September 6, 2007
Journal of Antimicrobial Chemotherapy 2007 60(5):1178-1179; doi:10.1093/jac/dkm343
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Correspondence |
Is Bordetella pertussis susceptibility to erythromycin changing? MIC trends among Australian isolates 1971–2006
1 Centre for Infectious Diseases and Microbiology, Institute of Clinical Pathology and Medical Research, Sydney West Area Health Service, Sydney, Australia 2 Western Clinical School, The University of Sydney, Sydney, Australia
* Corresponding author. Tel: +61-2-9845-6255; Fax: +61-2-9893-8659; E-mail: vitalis{at}icpmr.wsahs.nsw.gov.au
Keywords: pertussis , antibiotic therapy , resistance
Pertussis is a highly contagious respiratory tract infection caused by Bordetella pertussis and, less frequently, by Bordetella parapertussis. There is the suggestion that vaccination programmes over the last century may have led to genetic changes in B. pertussis.1 However, the extent of genetic diversity in different countries remains debatable. Macrolide antibiotics have been used for treatment and prophylaxis of pertussis for over 50 years, but occasional reports of resistance to erythromycin, the traditional antibiotic of choice, highlight the need for continued surveillance of pertussis.2,3 Unfortunately, widespread replacement of culture with direct nucleic acid amplification, for diagnosis of pertussis, has dramatically decreased the availability of clinical isolates for testing and resulted in a lack of data on the antimicrobial susceptibility of B. pertussis and its temporal trends. This study reports susceptibility to erythromycin of clinical isolates of B. pertussis collected from patients in New South Wales between 1971 and 2006.
A convenience sample of 99 isolates of B. pertussis and 5 of B. parapertussis from our laboratory collection was used. All isolates had been recovered from respiratory specimens by culture on selective agar plates and stored in nutrient broth containing 10% glycerol at –70°C. Suspensions of the isolates from a master stock were subcultured onto charcoal blood agar without antibiotics (Oxoid, Australia) and incubated in a moist, closed ambient air environment at 35°C for 72 h. The bacterial cells were then harvested and suspended in 2 mL of PBS and the turbidity adjusted to be equivalent to that of a 0.5 McFarland standard. All isolates were tested using 15 µg erythromycin discs. The plates were incubated at 35°C in high humidity, and zones of growth inhibition were measured on days 3 and 5. Isolates with growth inhibition of more than 40 mm in diameter after 3 days of incubation were considered as susceptible.4 Plates were incubated for 7 days to exclude heterogeneous phenotypes as indicated by resistant colonies appearing within the zone after extended incubation.5 The MIC of erythromycin was determined by Etest (AB BIODISK, Solna, Sweden), performed according to manufacturer's instructions. MICs were recorded on days 3 and 7 and interpreted as the lowest concentration of antibiotic that completely inhibited bacterial growth. Isolates with reduced susceptibility to erythromycin were tested further to determine their susceptibility to azithromycin by Etest.
We separated isolates into three groups based on the year of isolation as follows: 1971–1978 (12 isolates); 1995–2000 (51 isolates); and 2001–2006, after the introduction of acellular pertussis vaccine for routine infant immunization, in Australia (36 isolates).
The results are summarized in Table 1. All B. pertussis isolates were susceptible to erythromycin. Despite a slight increase in the median MIC in 2001–2006 compared with the earlier periods (Figure 1), the difference was not statistically significant and all MICs were <0.12 mg/L (the cut-off for susceptible strains).3,4 B. parapertussis isolates were less susceptible to erythromycin; the mean MIC (=0.25 ± 0.07 mg/L) was higher than for B. pertussis. This difference was statistically significant (P = 0.004, t = 6.053). B. parapertussis isolates also demonstrated reduced susceptibility to azithromycin with an MIC of 0.50 mg/L.
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Macrolide resistance in B. pertussis remains uncommon. To date, only four erythromycin-resistant isolates have been reported: the first was detected in Arizona in 1994 with others subsequently in Minnesota, Utah and California.2,3 Their resistance is probably due to modification of the erythromycin-binding site on the 23S rRNA of the 50S ribosomal subunit, resulting from an A-to-G transition mutation at position 2058.3 In pneumococci, high-level resistance [constitutive macrolide, lincosamide, streptogramin B resistance (MLSB) phenotype inducible by 14- and 15-membered ring macrolides] can be produced by erythromycin-resistant methylase encoded by a series of nine structurally related erm genes. Low-level resistance (typical erythromycin MIC 8–16 mg/L) may be displayed by bacteria carrying the chromosomal gene encoding macrolide efflux [the mef(E) and mef(A) genes or msr(A) genes]. We have found no indication that erythromycin MIC distributions in B. pertussis appear bimodal or trimodal, suggesting possible differences in resistance mechanisms.
This study of clinical isolates demonstrates no significant decrease in the susceptibility to erythromycin among B. pertussis strains isolated in Australia over the last 35 years. It supports previous observations from the USA, which failed to detect erythromycin resistance in clinical isolates recovered a decade after the initial reports of resistance.2,3,5 The re-emergence of pertussis in the Western world makes it important to note that B. parapertussis is generally less susceptible to macrolides than previously thought.6 Our findings confirm that routine susceptibility testing of B. pertussis is unnecessary, except for surveillance or when there is evidence of therapeutic failure.
No external funding was received for this study.
None to declare.
Acknowledgements
We thank Danny Ko, Maureen Lynch and Marion Yuen for their technical assistance and expert advice.
References
1
Mooi FR, van Oirschot H, Heuvelman K, et al. Polymorphism in the Bordetella pertussis virulence factors P.69/pertactin and pertussis toxin in The Netherlands: temporal trends and evidence for vaccine-driven evolution. Infect Immun (1998) 66:670–5.
2
Korgenski EK, Daly JA. Surveillance and detection of erythromycin resistance in Bordetella pertussis isolates recovered from a pediatric population in the Intermountain West region of the United States. J Clin Microbiol (1997) 35:2989–91.
3
Bartkus JM, Juni BA, Ehresmann K, et al. Identification of a mutation associated with erythromycin resistance in Bordetella pertussis: implications for surveillance of antimicrobial resistance. J Clin Microbiol (2003) 41:1167–72.
4
Hill BC, Baker CN, Tenover FC. A simplified method for testing Bordetella pertussis for resistance to erythromycin and other antimicrobial agents. J Clin Microbiol (2000) 38:1151–5.
5
Wilson KE, Cassiday PK, Popovic T, et al. Bordetella pertussis isolates with a heterogeneous phenotype for erythromycin resistance. J Clin Microbiol (2002) 40:2942–4.
6 Galanakis E, Englund JA, Abe P, et al. Antimicrobial susceptibility of Bordetella pertussis isolates in the state of Washington. Int J Antimicrob Agents (2007) 29:609–11.[CrossRef][Web of Science][Medline]
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