Typing of 154 clinical isolates of Stenotrophomonas maltophilia by pulsed-field gel electrophoresis and determination of the in vitro susceptibilities of these strains to 28 antibiotics

Franz-Josef Schmitza,b,*, Ralf Sadurskia, Jan Verhoefb, Dana Milatovicb, Ad C. Fluitb and the European SENTRY participants

a Institute for Medical Microbiology and Virology, Heinrich-Heine University Düsseldorf, Universitätsstraße 1, Geb. 22.21, 40225 Düsseldorf, Germany and b Eijkman-Winkler Institute, University Medical Center Utrecht, Utrecht, The Netherlands

Sir,

Stenotrophomonas maltophilia is an uncommon cause of nosocomial infections which is being reported with increasing frequency, particularly among immunocompromised patients.1 Risk factors for colonization and infection with this bacterium include previous exposure to broadspectrum antibiotics, prolonged hospitalization, mechanical ventilation and the use of intravascular devices.1,2 Uncertainty still exists, however, regarding the route(s) of acquisition.13 Strains of S. maltophilia are inherently resistant to most ß-lactam and non-ß-lactam antibiotics. Resistance to the former class of drugs is the result of permeability barriers and the production of ß-lactamases. Of the two recognized ß-lactamases, one is a zinc-dependent metalloenzyme which degrades carbapenems and is resistant to inhibition by ß-lactamase inhibitors, while the second is a cephalosporinase which is susceptible to inhibition by ß-lactamase inhibitors.1 The objectives of the present study were, first, to investigate the intra- and inter-hospital clonal relationships among S. maltophilia isolates collected by participants in the European SENTRY Antimicrobial Surveillance Programme and, secondly, to determine the in vitro susceptibilities of these strains to 28 antibiotics.

The organisms studied were 154 non-replicate strains isolated between April 1997 and February 1999 in 24 university hospitals participating in the European SENTRY Antimicrobial Surveillance Programme.4 The number of isolates recovered in each hospital ranged from one to 23. The strains were isolated from patients with septicaemias (81), nosocomial pneumonias (59) or wound infections (14). The hospital location of the patients from whom the greatest number of strains was isolated was the intensive care unit (64), while other patients were managed on general medical wards (54), surgical wards (10) and paediatric wards (nine).

All the isolates were typed by pulsed-field gel electrophoresis (PFGE) (GenePath Strain Typing System, BioRad, Hercules, CA, USA) following digestion with SpeI. The PFGE patterns were recorded with the Geldoc System (BioRad) and analysed with GelCompar (Applied Mathematics, Kortrijk, Belgium) by the UPGMA method.5 Two or more isolates were considered to be clonally related if their PFGE patterns exhibited >=80% similarity.5 MICs for the isolates were determined by a microbroth dilution method recommended by the National Committee for Clinical Laboratory Standards.6 The antibiotics tested are shown in the Table.Go


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Table. In vitro activities of 28 antibiotics against 154 S. maltophilia clinical isolates
 
PFGE typing of the 154 isolates demonstrated that 22 strains belonged to eight different PFGE types; the patterns of the remaining 132 isolates were unique. Between two and five clonally related isolates belonged to each of the eight PFGE types referred to above. The constituents of each of five pairs of such isolates exhibited the same PFGE pattern (two pairs from patients in one hospital and one pair from patients in a second hospital in Italy and one pair each from patients in hospitals in the UK and Turkey). Three clonally related strains (recovered from patients in a hospital in Italy) belonged to a sixth PFGE type, four strains (recovered from patients in a hospital in Spain) to a seventh and five strains (recovered from patients in a hospital in The Netherlands) to the eighth. All clonally related isolates were recovered from patients in the same hospital, thereby suggesting nosocomial transmission. Ten of the 22 clonally related isolates were recovered from patients with septicaemias, nine from patients with nosocomial pneumonias and three from patients with wound infections. With the exception of the five strains isolated from patients in a hospital in The Netherlands (three of which were isolated from patients with wound infections and two from patients with septicaemia), all clonally related isolates were recovered from the same specimen type.

Although small clusters of clonally related PFGE types were identified in this study, most of the patients harboured strains with unique patterns. Similar observations have been reported by other groups of investigators who also used PFGE, which has been shown to be a highly discriminating means of detecting clonality among isolates of S. maltophilia.2,3 Despite the increasing numbers of reports in the literature of outbreaks caused by this bacterium, attempts to identify the precise sources and modes of transmission have been largely unsuccessful.1,2 However, the great diversity of strains and the relative frequency of isolation from environmental sites suggest that the acquisition of multiple strains, independently of one another, from environmental sources is an important mode of transmission.3

The results of the susceptibility tests are summarized in the Table.Go Few studies have determined the in vitro susceptibilities of strains of S. maltophilia isolated from disparate locations.1 Most such studies, including our own, have shown co-trimoxazole to be the most active agent (98% of strains susceptible in this study) and this drug is regarded as the treatment of choice. However, because it exhibits only bacteriostatic activity against the majority of isolates, it has been proposed that patients should receive a combination of co-trimoxazole and either ticarcillin–clavulanate or an extended-spectrum cephalosporin such as ceftazidime.1 The latter compounds exhibit moderate activities against S. maltophilia (84.5% and 77.4% of isolates, respectively, being susceptible in the present study) and, indeed, were the most active of the ß-lactams evaluated by us. A recent study by Lesco-Bornet & Bergogne-Bérézin7 demonstrated that ticarcillin–clavulanate was the only ß-lactam– ß-lactamase inhibitor combination to exhibit synergic activity against this bacterium; more than 80% of the strains evaluated in that study were susceptible to the combination and more than 50% exhibited synergic activity. The activities of tetracycline and the aminoglycosides in the present study were disappointing. Amikacin was the most active aminoglycoside, but only 62.6% of the strains tested were susceptible to it. In general, aminoglycosides and tetracyclines have limited roles in the treatment of patients with infections caused by strains of S. maltophilia. Of the quinolones, the more recently introduced compounds were markedly more active than earlier agents belonging to this class. Clinafloxacin and sitafloxacin were the most active drugs tested by us, 99.1% and 100% of isolates, respectively, being susceptible. Although this and other in vitro studies suggest that quinolones show promise as therapeutic agents, there is a paucity of data currently available concerning their use as therapy for patients with infections caused by S. maltophilia. The results of clinical trials are therefore awaited with interest.1 For the time being, and because of the often disappointing clinical outcomes associated with monotherapy, treatment should normally comprise a combination of drugs.1

In summary, PFGE typing of recent clinical isolates of S. maltophilia recovered from patients in 24 university hospitals in Europe revealed that the strains belonged to a wide diversity of types. Although small clusters of clonally related strains were identified, the isolates from most patients were unique. On the basis of our in vitro results, a combination of co-trimoxazole and either ticarcillin–clavulanate or ceftazidime currently represents optimal therapy. None the less, the potential of other therapeutic agents, particularly the newer quinolones, warrants further investigation.

Acknowledgments

The authors thank Miriam Klootwijk, Carlijn Kusters, Alice Florijn and Stefan de Vaal for expert technical assistance. This study was funded partly by the SENTRY Antimicrobial Surveillance Programme, which itself is funded by an educational grant from Bristol-Myers Squibb Pharmaceutical Co., and partly by the European Network for Antimicrobial Resistance and Epidemiology (ENARE) with a grant (ERBCHRCT940554) from the European Union.

Notes

J Antimicrob Chemother 2000; 45: 921–924

* Corresponding author. Tel/Fax: +49-2132-72040; E-mail: schmitfj{at}uni-duesseldorf.de Back

References

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