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Community-acquired bacterial pathogens
in Singapore: changes and challenges
Moi-Lin Ling
Department of Pathology, Singapore General Hospital, Singapore
Like many developing nations, Singapore is currently faced with an alarming increase in the resistance rates of
common pathogens responsible for potentially life-threatening, community-acquired infections. These increases have
been noted in Streptococcus pneumoniae, as well as in the beta-lactamase producing organisms Haemophilus influenzae
and Moraxella catarrhalis. Many Salmonella species are also showing an increase in resistance rates, posing unique
sets of problems for the clinician and patient.
Respiratory bacterial pathogens
The most striking change Singapore has seen is in the incidence of penicillin-resistant S. pneumoniae. A study
done between 1986 and 1997 of penicillin-susceptible invasive isolates at the Department of Pathology, Singapore
General Hospital showed only a 0.5% rate of intermediate resistance. (1) That rate began a dramatic ascent, from
2% in 1991, to 15.4% in 1995, 36.3% in 1996, and 43% in 1997. In 1998, the rate of penicillin resistance in S.
pneumoniae stood at 26%, with ceftriaxone resistance at 20%. Changes in the rates of resistance to other antimicrobials
have also been noted in this organism (Fig.1). Significant increases in resistance to erythromycin, tetracycline
and chloramphenicol are also posing persistent problems in Singapore, not unlike the multidrug resistance seen
elsewhere in the world.
Alarming increases in the resistance rates of other strains have
also been noted. H. influenzae, for instance, has seen nearly a two-fold increase, from 18% in 1995, to 32.8% in
1998 in respiratory isolates, and to 66.7% in 1998 in invasive isolates. Ampicillin resistance in M. catarrhalis
has increased from 68.4% in 1995 to 92.7% in 1998. As this drug is used by most general practitioners for the empirical
treatment of respiratory tract infections, this increase is of particular concern.
Of positive note is the decrease in the number of tuberculosis cases, from 307 cases per 100,000 in 1960, to 57.2
cases per 100,000 in 1998. Multidrug resistance has remained low (0.38%), due in large part to Singapore’s commendable
tuberculosis control program.(2)
The challenge now lies in the management of pneumococcal infections,
especially in light of high ceftriaxone resistance. The newer quinolones, grepafloxacin and levofloxacin, were
introduced to the Singapore market in 1998 and 1999, and they are now alternative drugs for the treatment of penicillin-
and ceftriaxone-resistant strains. Judicious use of these newer antimicrobials is imperative if we are to maintain
their efficacy.
Gastrointestinal bacterial pathogens
Singapore’s Department of Pathology is also its national center for the surveillance of enteric pathogens. In 1998,
they received 1,093 strains of Salmonella species for serotyping from seven public restructured hospitals. Of these,
the most common species were Salmonella enteritidis (19.7%), S. typhimurium (14.7%) and S. stanley (13.4%). The
number of S. enteritidis isolates increased from 0% in 1991 to 53.4% in 1993. (data were missing for the year 1992;
see Fig. 2). In 1998, 67.7% of all S. enteritidis isolates were phage type 4, followed by phage type 1 at 14.9%.
In the case of S. typhimurium phage type U302 was most common (49.2%), followed by phage type 151 (21.3%) and phage
type 104L (13.1%). Fortunately, while the species may change in numbers, their rates of susceptibility to common
drugs remain high. In 1998, for instance, all Salmonella strains were susceptible to amikacin, 99.6% were susceptible
to ceftriaxone and 99.5% were susceptible to ciprofloxacin.
The changes in epidemiology of S. enteritidis is an interesting
phenomenon, following those seen in other countries. However, the direct source of this organism has yet to be
determined. S. typhimurium phage type DT104 is still rarely isolated in Singapore. S. typhi have been isolated
from travelers entering into, or returning to, Singapore from overseas. Hence, they reflect the epidemiology of
strains from regions in Southeast Asia. Phage type E1 is the most common isolate, comprising 35.4% of all organisms
in 1998. All the strains were susceptible to ceftriaxone and ciprofloxacin; however, 4.2% were resistant to ampicillin,
chloramphenicol and co-trimoxazole.
Antibiotic policy
The evolution of resistant bacteria in Singapore continues to pose complex challenges to clinicians and policy-makers
in our country. Therapeutic options become more limited and constrained in the face of increased antimicrobial
resistance. In view of this, steps are actively being taken by the Ministry of Health, advocating all hospitals
to draw up written policies, procedures and guidelines for prescribing antibiotics. Hopefully, these measures will
prove to be but the beginning of many steps toward sound policies and judicious habits.
References
- Ling ML, Tay L. 1990. Ann Acad Med 19: 777-780.
- Communicable Disease Surveillance Report. 1998. Department
of Clinical Epidemiology, Communicable Disease Centre, Tan Tock Seng Hospital.
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