M002: MOLECULAR EPIDEMIOLOGY OF PSEUDOMONAS AERUGINOSA INFECTION
IN CYSTIC FIBROSIS PATIENTS
M.J. Struelens, Microbiology and Infection Control Departments,
Hopital Erasme, Universite Libre de Bruxelles, Bruxelles, Belgique.
Pseudomonas aeruginosa remains a major cause of chronic morbidity and reduced life expectancy in patients with cystic fibrosis (C.F.). Because host-adapted strains of P. aeruginosa recovered from the respiratory tract of these patients show similar or unstable phenotypic characteristics, the study of the epidemiology of this chronic infection cannot rely on phenotypic markers. In recent years, several molecular typing methods have enabled progress in this area: chromosomal restriction fragment - length polymorphism (RFLP) by using the toxA gene probe (1), genome macrorestriction analysis resolved by pulsed field gel electrophoresis (2-3) and arbitrarily-primed PCR fingerprinting (4).
Long term prospective studies using macrorestriction analysis have shown that a majority of CF patients harbour a single P. aeruginosa clone for many years, and less commonly a second or third clone for transient periods of time (2-3). In one study, increasing antimicrobial resistance was associated with true emergence of resistant variants in about 80% of the cases and superinfection with a distinct clone in 20% of the cases (3). Revertants to susceptible phenotypes also occurred commonly- among clonal variants. Subclonal variation at the genomic level can be detected over several years of carriage, with shifts of macrorestriction patterns yielding up to 20% pattern diversity (3-4).
In several recent studies, epidemiologically related CF patients were found to carry identical P. aeruginosa clones in their lungs, including among patients attending the same clinic (2-5), rehabilitation centre (2) or holiday camps (1). This finding may indicate: (i) cross-infection during hospital care or during social contact between CF patients, (ii) common-source infection from a nosocomial or other environmental reservoir; (iii) independent acquisition from multiple sources of widespread clones colonizing natural or man-made environment.
Direct evidence of cross-infection has been gathered by Wolz et al, who documented a risk of 9% cross-colonization among CF patients attending a 6-week long recreation camp (1). In studies of 1-week long holiday camps, we observed an attack-rate of 14% cross-infection during 42 available patient stays. However, long-term follow-up was not available to assess the persistence and clinical relevance of this phenomenon. In studies in a rehabilitation centre, we were unable to detect cross-infection in 23 CF patients over 8-weeks of study, suggesting that the rate of transmission was low in a centre where maximal hygienic precautions were taken.
A study by Romling et al clearly illustrate the possibility of independent acquisition of the same clone by CF patients from multiple sources in the aquatic environment (5). Closely related clonal variants were identified in 28%
CF patients living in one city and 21% of water samples collected 300 km away from the CF clinic (5).
In conclusion, genomic typing systems have provided new insights into the epidemiology of P. aeruginosa infection in CF patients. Antibiotic resistance appears predominantly associated with mutant selection rather than nosocomial transmission. Cross-infection occurs at a rate of 1 case per 100 days of stay at CF recreation camps, but appears less common in clinics or rehabilitation centres with good hygiene standards. The role of the environmental reservoir outside the hospital as a source of infection may be important and deserves further study.
References
1. Wolz C., G. Kiosz, J.W. Ogle, M.L. Vasil, U. Schaad, K. Botzenhart, and G. Döring. 1989. Pseudomonas aeruginosa cross-colonization and persistence in patients with cystic fibrosis. Use of a DNA probe. Epidemiol. Infect. 102:205-214.
2. Grothues D., V. Koopman, H. von der Hardt, and B. Tümmler. 1988. Genome fingerprinting of Pseudomonas aeruginosa indicates colonization of cystic fibrosis siblings with closely related strains. J. Clin. Microbiol. 26:115-123.
3. Struelens M., V. Schwam, A. Deplano and D. Baran. 1993. Genome macrorestriction analysis of diversity and variability of Pseudomonas aeruginosa strains infecting cystic fibrosis patients. J. Clin. Microbiol. 31 :2320-2326.
4 Kersulyte D., M.Struelens, A. Deplano and D.E. Berg. 1995. Comparison of arbitrarily primed PCR and macrorestriction (pulsed field gel electrophoresis) typing of Pseudomonas aeruginosa strains from cystic fibrosis patients. J. Clin. Microbiol. Submitted.
5 Römling U., J. Wingender, H. Muller and B. Tümmler. 1994. A major Pseudomonas aeruginosa clone common to patients and aquatic habitats. Appl. Environ. Microbiol. 60:1734-1738.