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Abstract

MJA 1996; 164: 64  

Introduction

It was from Australia in 1967 that the first clinically significant isolate of a penicillin-resistant pneumococcus was reported.⁵ However, penicillin resistance was not a major clinical problem in this country, although it caused major problems elsewhere, particularly in Papua New Guinea and South Africa.^1-3 In the late 1970s and the 1980s, rates of resistance (including multiple resistance) increased in Western countries, particularly in Spain (with resistance levels of 50%).^1,2,3 A recent United States study found 25% of invasive S. pneumoniae isolates were penicillin-resistant.⁶ Resistance rates are usually higher in children, and the distribution of resistance varies within countries and population groups.^1,2,3,6

In an Australia-wide study of over 1800 isolates of S. pneumoniae in 1989, we found that only 1% were penicillin-resistant,⁷ a lower rate than in most other Western countries. However, some communities (especially Australian Aboriginals) have relatively high rates of resistance.⁸

Because of the worldwide increase in resistance to many antibiotics and the implications of penicillin resistance in S. pneumoniae for treatment of life-threatening conditions (particularly meningitis), we undertook a further study of resistance to penicillin and other commonly used antibiotics in clinically significant S. pneumoniae isolates from both the community and hospitals.  

Methods

Patients' sex, age, specimen site and inpatient or outpatient status were recorded prospectively.

Clinically significant isolates were defined as those isolated either from normally sterile sites (e.g., cerebrospinal fluid and blood [invasive isolates]) or from specimens that made contact with mucosal surfaces (e.g., sputum) if they were associated with an increased white cell count on gram staining and would normally have been reported as clinically significant. Throat or surveillance swabs were excluded, as were duplicates of clinically significant isolates.

S. pneumoniae was identified by colonial morphology, a -haemolysis on blood agar plates, susceptibility to optochin and/or bile solubility.  

Antibiotic sensitivity testing

The MIC of penicillin was also determined for each isolate by the Etest on Mueller-Hinton agar supplemented with 5% blood;^13,14 plates were incubated at 35¡C in 5% CO ₂ for 20-24 hours. ¹⁴ The interpretive criteria of the NCCLS¹⁵ were used for susceptibility categorisation of Etest values (susceptible, MIC < 0.06 mg/L; intermediate resistance, MIC = 0.125-1 mg/L; and high level resistance, MIC > > 2 mg/L).

For the study, antibiotic resistance was defined as decreased susceptibility (both intermediate and high level resistance),^6,12 and multidrug resistance as decreased susceptibility to two or more of the antibiotic agents tested.

Isolates from normally sterile sites and those that appeared resistant to penicillin or chloramphenicol by routine susceptibility testing or had penicillin MICs > > 0.047 mg/L were forwarded to Monash Medical Centre for further susceptibility testing: Etest strips were used to determine cefotaxime and ceftriaxone MICs.  

Statistical analysis

Results

The percentage of penicillin-resistant isolates from each specimen site is shown in Box 1; the overall rate of penicillin resistance was 6.7%, with rates in individual laboratories ranging from 0 to 13%. High level resistance was seen in 17 isolates, including two from normally sterile sites.

The rate of penicillin resistance was significantly lower among invasive isolates than among non-invasive isolates (3.7% versus 7.6%; odds ratio [OR], 0.47; 95% confidence interval [CI], 0.28-0.77; P = 0.001) (Box 2). The rate of penicillin resistance was slightly higher among children ( < 15 years) than among adults, but the difference was not statistically significant (7.3% versus 6.5%; OR, 1.14; 95% CI, 0.8-1.63; P = 0.47).

Resistance to antibiotics other than penicillin was common (Boxes 2 and 3). Rates varied around Australia, with the lowest rates for nearly all antibiotics in Tasmania and the highest in the eastern States, particularly Queensland and New South Wales. The rate of penicillin resistance was highest in South Australia. Very high levels of resistance were seen for trimethoprim-sulfamethoxazole (29%-52%).

All five antibiotics were tested on 1895 isolates; 267 (14%) were multi resistant, with 159 (8%) resistant to three or more antibiotics and 31 (1.6%) to all five (Box 3). Of 124 penicillin-resistant isolates tested, 72 (58%) were resistant to three or more non--lactam agents; 40% were resistant to chloramphenicol; 52% to erythromycin; 64% to tetracycline; and 78% to trimethoprim-sulfamethoxazole. Of the 1771 pencillin-susceptible isolates, 101 (6%) were resistant to three or more non--lactam agents; 3% to chloramphenicol; 8% to erythromycin; 12% to tetracycline; and 39% to trimethoprim- sulfamethoxazole .

Of 109 penicillin-resistant isolates tested with cefotaxime, 17 (16%) had intermediate resistance (MIC, 1 mg/L). These comprised 12 of 13 isolates with high-level penicillin resistance and 5 of 96 with intermediate penicillin resistance. Only three of the 109 isolates had intermediate resistance to ceftriaxone (all with high-level penicillin resistance).  

Discussion

The finding of high level penicillin resistance among S. pneumoniae isolates in Australia is of particular concern; in meningitis caused by organisms with any level of penicillin resistance, penicillin treatment is likely to fail.^1-3,17,18 Penicillin resistance has consequences for other related drugs, as in S. pneumoniae it is not due to -lactamase production (as in Staphylococcus aureus ), but to changes in the target for penicillin (the penicillin-binding proteins).^1,2,19 This change increases the MICs for all -lactams, including the third generation cephalosporins.^1,2,19 However, the levels of third generation cephalosporins achieved in cerebrospinal fluid are still high enough to eradicate organisms with intermediate penicillin resistance.^1-3,18 Alternative regimens include combination therapy with vancomycin, third generation cephalosporins and rifampicin, as well as newer agents such as meropenem, teicoplanin and quinolones (under investigation),¹ but none has been adequately evaluated.

Of even greater concern are the implications of the rapid rise in resistance to third generation cephalosporins noted in the United States. Primary resistance to these drugs is less frequent than to penicillin, but requires less genetic change.^1,19 In some areas up to 27% of penicillin-resistant pneumococci have high level resistance to cefotaxime.¹ This leads to therapeutic failure of these agents, yet they are the main treatment for the increasingly common intermediate penicillin-resistant strains. No high level cefotaxime-resistant strains were seen in our study or have been reported in Australia, to our knowledge. However, given the worldwide spread of resistant pneumococci in the recent past, they will inevitably be seen soon in Australia and leave us with major therapeutic dilemmas in the treatment of meningitis.

In life-threatening situations other than meningitis (e.g., bacteraemia), high dose intravenous penicillin appears sufficient to eradicate organisms with intermediate resistance, as drug levels achievable in serum are still much higher than the MIC.^1,2,3 There is, however, controversy, and many recommend use of either cefotaxime or ceftriaxone.^1,3 For organisms with high level resistance, the most appropriate agent is unclear. However, we would favour vancomycin.

In non-life-threatening infections with penicillin-resistant pneumococci, the most appropriate antibiotics are less clear. In otitis media, amoxycillin still appears the best choice,^20,21 as drug levels achieved in the middle ear can still exceed the MICs of strains with intermediate resistance (although higher doses may be needed). Other oral agents available in Australia for use in children (cefaclor, trimethoprim, erythromycin and cefpodoxime) do not reach adequate levels to eradicate resistant isolates.^20,21 Third generation cephalosporins, such as ceftriaxone, are active, but their parenteral route is likely to preclude their use. Combining clavulanic acid with amoxycillin is no advantage, as the resistance is not due to -lactamase. For high level penicillin-resistant isolates there does not appear to be a satisfactory oral agent.

The reasons for the increasing resistance in S. pneumoniae worldwide are not completely understood, although antibiotic pressure appears to be a major factor.¹ A few resistant clones were shown to have spread from one continent to others (e.g., from Spain to the United States and Iceland) and then through the local population, undergoing minor genetic changes in the process.^1,3,19 The pneumococcus can acquire DNA molecules from other bacteria that probably include viridans group streptococci (e.g., Streptococcus mitis), which form part of the normal flora of the nasopharynx.^1,19 While it would probably be impossible to eradicate carriage of these resistant organisms from the population, it may be possible to reduce the rate of increase in resistance by minimising the prescription of unnecessary antibiotics.

Other strategies, such as vaccination, may be necessary. Unfortunately, the currently available vaccine is a polysaccharide and therefore a poor immunogen, especially in young children. Studies are under way to assess a conjugated pneumococcal vaccine (i.e., a carbohydrate with protein carrier), but vaccine development is difficult as there are over 80 serotypes of pneumococci (compared with only one commonly invasive serotype of Haemophilus influenzae -- type b). However, at present most of the resistant organisms belong to relatively few serotypes.^1,2,3 A vaccine containing most of these might not only decrease life-threatening disease, but might also decrease carriage of the organisms, as was found for the H. influenzae type b (Hib) vaccine.¹ However, as the pneumococcus can acquire DNA from other organisms,^1,19 the number of resistant serotypes is likely to increase.

Our study was one of the largest in the world where all organisms were clinically significant and all were assessed for MIC for penicillin. It is valuable not only for showing the rate of resistance (both intermediate and high level), but also for providing a baseline to assess future changes in resistance and to differentiate subtle shifts in resistance in the whole population of pneumococci from the introduction of resistant clones.

In the past, determining MICs was time-consuming, laborious and not routine. The recent development of the Etest (which consists of a strip of paper impregnated with increasing concentrations of antibiotic from one end to the other) has simplified the procedure. This technological advance, along with the willingness of so many laboratories around Australia to participate in the project, has allowed us to obtain information essential for guiding us in making appropriate antibiotic choices and designing empiric therapy for these emerging threats.  

Acknowledgements

References

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