Antibiotic resistance in Haemophilus influenzae in Portugal
Bajanca Lavado and Vaz Pato
Instituto Nacional de Saude Dr. Ricardo Jorge, Lisboa, Portugal
In recent years, resistance of clinical isolates of Haemophilus influenzae
to a variety of antimicrobial agents such as ampicillin, chloramphenicol, tetracycline and cotrimoxazole has been
reported in many countries (1,2).
Ampicillin resistance in Haemophilus influenzae is often due to the plasmid-mediated production of TEM b-lactamase, altered penicillin-binding proteins and/or diminished permeability
Chloramphenicol resistance caused by chloramphenicol acetyl transferase (CAT) is encoded on large (30-50 Mdal)
plasmids, which may be easily transferred by conjugation and commonly integrate into the chromosome. Sometimes
these plasmids also encode for resistance to tetracycline and ampicillin (3).
The prevalence of antimicrobial resistance of 374 isolates of Haemophilus
influenzae collected through a Multicentric Surveillance
Study in Portugal, between January and December 1992 was examined. The sources (with number of isolates) included
respiratory tract (307), body fluids (19), head (11), skin (3) and miscellaneous (34).
Clinical specimens were cultured on chocolate agar plates and incubated
overnight at 37°C in 5% CO2. Organisms were identified as Haemophilus
influenzae on the basis of colony characteristics, Gram
staining morphology and requirement for factors X and V. Strains were assigned to one of the eight biotypes on
the basis of indole, urease and ornithine decarboxylase production (4). The prevalent biotypes were I, II and II,
with 25.9, 34.2 and 21.4% of the strains, respectively. Encapsulated strains accounted for 4.6% of the total; 70.6%
of these were isolated among invasive strains (cerebrospinal fluid, blood and bronchial aspirate) and 64.7% belonged
to biotype I; all but one were serotype b.
Susceptibility to ten antibiotics (ampicillin, amoxicillin/clavulanate,
cefotaxime, ceftriaxone, ciprofloxacin, chloramphenicol, erythromycin, rifampicin, tetracycline and trimethoprim)
was assessed by the agar dilution method, with an inoculum of 104 cfu (5) (Table 1). All strains were susceptible
to cefotaxime, ceftriaxone, ciprofloxacin, rifampicin and the amoxicillin/clavulanate combination. As expected,
most minimum inhibitory concentrations (MICs) for erythromycin (93.6%) fell into the NCCLS intermediate category
(1-4 sg/ml) (5,6).
Chloramphenicol resistance was uncommon (2.1%) but was associated with ampicillin resistance in 87.5% and with
tetracycline resistance in all of the isolates. Multiple resistance occurred in 7 of the 374 Haemophilus isolates (1.9%), the most common pattern being resistance to ampicillin,
chloramphenicol and tetracycline. A conjugative plasmid of 30-36 Mdal was observed in 43% of these multiresistant
strains (Figure 1).
By b-lactamase production, levels of ampicillin resistance in Haemophilus influenzae
in Portugal have been reported previously: 4.3% of 150 isolates in 1982, in the National Institute of Health in
Lisbon (7) and 11.3% of 248 isolates in 1987-1988 in an Hospital of Oporto (8). More recently, we have observed
a level of 18% in a survey of 258 isolates collected in 1989 in Lisbon from Hospital and from the National Institute
of Health (9). At that time this result represented a much higher frequency of resistance than reported in 1982.
In the present study, the frequency of ampicillin resistance was
noted to be 10.4%, which is in agreement with the average found in two previous European cooperative studies: 10.9%
in 1986 (10) and 9.1% in 1988-1989 (11). Nevertheless, resistance to ampicillin seems to be confined to some areas.
A wide variation was observed in 1986 in relation to country. Some countries showed a relatively low percentage
strains (Austria: 5.9%, FRG: 1.6%, Netherlands: 6.8%, Switzerland: 6.0%), while others a high percentage (Belgium:
26.7%, France: 10.9%). In Spain, our only neighbor country, the frequency observed for ampicillin resistance in
1990 was much higher, 35% (3). Also, higher rates were reported in the USA in 1986 (20%) and in Asia: Hong-Kong
with 20% and Taiwan with 35.7% (1).
Many factors probably contributed to the differences among the various
countries: characteristics of the populations studied, variations in the mode of collection of isolates, source
of the specimens and antibiotic usage. The frequency of conjugative plasmids specifying antibiotic resistance may
also influence the prevalence of resistance to certain antimicrobial agents. Thus, the choice of antibiotic therapy
for infections caused by Haemophilus influenzae should be based on knowledge of the local prevalence of antibiotic resistance.
Recognition of antimicrobial resistance and awareness of its prevalence in Haemophilus
influenzae must be continued in our country.
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