Emerging resistance among bacterial pathogens in the intensive care unit – a European and North American Surveillance study (2000–2002)

Background

Antimicrobial resistance has emerged as an important factor in predicting outcomes and overall resource use after infections in intensive care units (ICU) [1]. Globally ICUs are encountering emergence and spread of antibiotic-resistant pathogens. For some pathogens there are few therapeutic options available, e.g., vancomycin-resistant Enterococcus faecium. Awareness of these problems has been underscored with data from a number of surveillance studies aimed at improving the use of empiric therapy. In the United States there have been several national programs, which have focused on both the etiology of infections and resistance patterns of nosocomial or ICU infections including the National Nosocomial Infections Surveillance (NNIS) [2] and more recently an ICU-specific study examining the epidemiology of antimicrobial resistance, Project ICARE [3,4]. Stephen et al. collected strains from 28 ICUs from across the United States as part of the SENTRY Antimicrobial Surveillance Program in 2001 [5].

European data on the antimicrobial resistance of ICU pathogens has also been collected in several recent surveillance studies. A large prevalence survey of nosocomial infections in ICUs in 17 countries was published in 1995 [6], and more recently a number of nation-specific surveys were reported [7-9]. Several key points emerge: first, antimicrobial resistance among ICU pathogens is generally increasing, but variations do exist among different countries, probably due to individual antimicrobial use patterns; second, when new medical practices and alternative antimicrobials are introduced changes in the dominant microbial etiologies may emerge prompting novel empiric selections; and third, the standards of hygiene and infection control also vary across countries. Finally, appropriate therapy of ICU infections directed by local resistance data can have significant consequences for both patient and the healthcare system. It is against this background that local resistance surveillance programs are of most value in developing appropriate therapeutic guidelines for specific infections and patient types. For example, the recent modification to the American Thoracic Society guidelines for the treatment of hospital-acquired pneumonia [10] considered contemporary resistance data. Local surveillance data can be applied to other infections to assist in local formulary policy such as those governing treatment of nosocomial urinary tract infections [11].

This study using TSN program reports the antimicrobial resistance profiles of bacterial isolates from ICU patients in five countries during the period 2000–2002. The relevance of these recent nation-specific data will be discussed on a country-by-country basis, as part of improving and updating empiric therapeutic approaches to specific pathogens causing infections in the ICU setting according to each country. These surveillance programs help to maintain current knowledge of susceptibilities and relevant treatment options.

Methods

Results

In vitro susceptibility data from over 220,000 isolates from ICUs in five countries over the period 2000–2002 were assimilated. The most frequent species isolated from infections in the ICU was S. aureus, being most common in three of the five countries (Table 1). The oxacillin resistance rates among S. aureus varied markedly across countries from 19.7% in Canada to 59.5% in Italy. E. coli (7.7%–15.5%) and P. aeruginosa (10.8%–22.3%) were the most frequent Gram-negative organisms encountered. The Gram-positive genus Enterococcus, either as E. faecalis, E. faecium or non-speciated isolates accounted for <10% of isolates in most countries with E. faecalis being the most common species <4.3%. Community-acquired respiratory pathogens such as Streptococcus pneumoniae and Haemophilus influenzae were relatively uncommon in all five countries.

Tables 2,3,4,5 show the antimicrobial susceptibility profiles of various Gram-positive and Gram-negative pathogens isolated from ICU patients against a range of relevant antimicrobials.

Specifically notable susceptibility patterns include the vancomycin susceptibility of all strains of staphylococci. Generally, there was a low proportion of vancomycin resistant E. faecalis <5%, whereas vancomycin-resistant E. faecium was more prevalent ranging from 0.8% in France to 76.3% in the United States, with a wide inter-country variation (Table 2). Penicillin resistance rates varied among S. pneumoniae, from 2.0% in Germany to 20.2% in the US with concurrent ceftriaxone resistance rates of 0% in Germany to 3.4% in Italy (Table 3).

β-lactam activity was assessed by comparing four different cephalosporins and a β-lactam/β-lactamase inhibitor combination, piperacillin-tazobactam. Overall, the putative production of ESBLs among E. coli was low, <6%, but ceftazidime resistance was reported at higher rates in K. pneumoniae and S. marcescens, with the highest rates seen in M. morganii, from 16.0% in Germany to 26.4% in the United States (Table 4). Among the gram-negative organisms tested, ceftriaxone resistance rates were usually lower than ceftazidime, with the exception among P. aeruginosa and Acinetobacter spp. Cefepime, a fourth generation cephalosporin with anti-pseudomonal activity was also more active than ceftazidime (Table 5). Against the Enterobacteriaceae, the β-lactam combination agent piperacillin-tazobactam was generally less active than ceftriaxone. These species showed a wide variation in fluoroquinolone susceptibility among both species and countries. Gentamicin resistance rates among the Enterobacteriaceae varied from 1.2% among K. oxytoca from Germany to 37.2% in P. mirabilis from Italy. Ciprofloxacin resistance rates among E. coli ranged from 6.5% in France to 12.7% in Italy. Variable fluoroquinolone resistance rates among S. marcescens were also demonstrated, with a range of resistance from 4.5% in Italy to 12.4% in Germany.

Discussion

Data derived from international surveillance studies, such as those presented here, can provide a unique contemporary perspective on the susceptibility of commonly encountered organisms to commonly used antibiotics. Such surveillance systems play a crucial role in detecting emerging trends in resistance. Comparisons of these with data of other recent surveillance programs show the wide variations in susceptibility profiles and the need for ongoing unit-specific surveys.

In Germany the prevalence of resistance among gram-positive organisms remained comparatively low with an incidence of 21% MRSA. In 2000, Frank et al. reported that 96% of German isolates of S. marcescens and M. morganii were susceptible to ceftazidime, yet in this study we found 89.7% and 84.0%, respectively [9]. A similar decrease in activity was noted with E. coli and ciprofloxacin between the two studies, 91% in 1996–1997 compared with 86.7% in this study. Marked decreases in susceptibility of P. aeruginosa in Germany were also evident, with no agent showing >85.8% susceptibility (piperacillin-tazobactam) compared with most agents having 85%–94% susceptibility in 1996–1997. Changes of 15–20% have been reported with ceftazidime, imipenem, ciprofloxacin and meropenem, while piperacillin-tazobactam has shown the smallest decrease in susceptibility with <6% over the 4-year period. Piperacillin plus or minus tazobactam and cefepime were the most active agents, based on susceptibility, against P. aeruginosa in Germany. Conversely, ceftriaxone and imipenem were the most active agents, based on susceptibility, against Klebsiella spp., which account for almost 8% of ICU isolates.

Staphylococcal species from French ICU isolates showed a high proportion of oxacillin resistance, 40.6% and 69. 9% of S. aureus and coagulase-negative staphylococci spp., respectively. S. pneumoniae showed penicillin resistance of 17.9%, higher than the other four countries, although the activity of third-generation cephalosporins, ceftriaxone and cefotaxime, showed only 0.6% and 0.8% resistance, respectively. Despite a lower ceftazidime susceptibility breakpoint compared to NCCLS standards (MIC 4 μg/ml instead of 8 μg/ml) putative ESBL expression were slightly lower in France than in Germany in 2000–2002. Ceftazidime non-susceptibility rates among E. coli, K. oxytoca, and P. mirabilis were ≤ 2.2%; however, ceftazidime non-susceptibility rates among K. pneumoniae, M. morganii and S. marcescens were 7.5%, 21.4%, and 5.3%, respectively. Imipenem was active against all Enterobacteriaceae. Against P. aeruginosa and Acinetobacter spp., imipenem resistance rates were 21.4% and 3.8%, respectively. Previously, a lower imipenem resistance of 24% among French isolates of P. aeruginosa was reported [7].

Among the Italian isolates of staphylococci, oxacillin resistance occurred in 59.4% of S. aureus and 84.8% of coagulase-negative isolates. This MRSA rate was similar to that reported by Frank et al. from bacteremic isolates in Italy; however, they reported an increase in MRSA from 25% to 55% over the period 1997 to 2001 [18]. Vancomycin resistance rates of 2.8% for E. faecalis and 24.2% for E. faecium are some of the highest rates recorded in Europe, although still modest compared to rates experienced in the United States; however, teicoplanin was more active with 2.4% and 13.7% of strains being resistant, respectively. Pneumococcal resistance to penicillin and erythromycin was 7.6% and 28.1%, respectively. The impact of alterations in penicillin-binding protein that reduce penicillin susceptibility have less effect on the activity of third-generation cephalosporins such as ceftriaxone with 3.4% and cefotaxime with 4.6% resistance, respectively. S. pyogenes was fully susceptible to penicillin; however, 11.8% of isolates were resistant to clarithromycin and 23.7% were resistant to erythromycin.

The proportion of ESBLs was slightly higher in Italy with E. coli showing ceftazidime non-susceptibility of 5.3%, whereas K. pneumoniae and K. oxytoca demonstrated 30.2% and 16.6% ceftazidime non-susceptibility, respectively. Fluoroquinolone resistance rates among the Enterobacteriaceae, using ciprofloxacin as a marker, varied from 3.0% for K. oxytoca to 22.7% for P. mirabilis, and 12.7% for E. coli. Thus, among Enterobacteriaceae, ciprofloxacin was generally less active than the third-generation cephalosporin, ceftriaxone. P. aeruginosa and Acinetobacter spp. strains from Italian ICUs demonstrated significant resistance rates. Isolates of P. aeruginosa showed resistance rates of >28% for all agents tested except piperacillin-tazobactam. Thus empiric therapy for possible pseudomonal infections will require combination therapy. Acinetobacter spp. showed a similar lack of susceptibility except to imipenem and meropenem (19.0% and 13.6% resistant). An increase in fluoroquinolone resistance in E. coli and K. pneumoniae in bacteremic isolates from Italy was observed during 1997–2001, with rates of 26.7% and 24%, respectively [9]. An increase in ureidopenicillin resistance was noted in P. aeruginosa isolates in Italy from 30% to 37% in a 4-year period [9]. This study showed 22.0% piperacillin-tazobactam and 36.7% piperacillin resistance among ICU P. aeruginosa isolates.

In Canada oxacillin-resistance among S. aureus was noted in 19.7% and coagulase-negative staphylococci in 79.4%. Vancomycin resistance was reported among 0.9% and 14.5% of E. faecalis and E. faecium, respectively. The lowest rate of penicillin resistance in S. pneumoniae in this study was noted from Canada at 7.1%; however, clarithromycin resistance was 30.4%. Ceftriaxone showed 0.7% resistance whereas cefepime exhibited 12.0% resistance among pneumococci from the ICU.

Overall the susceptibility rates for Gram-negative isolates from Canadian ICUs were higher than those in the other four countries examined. A low rate of ESBLs was reported, but there was variable activity of piperacillin-tazobactam which showed >9% resistance among Klebsiella spp. and S. marcescens tested. The rate of fluoroquinolone resistance was similar to those of other countries with E. coli showing 13.9% levofloxacin resistance. Among Enterobacteriaceae, <10% of most species were resistant to third-generation cephalosporins tested with the exception of ceftazidime and M. morganii. Resistance among P. aeruginosa and Acinetobacter spp. was generally lower than in other countries apart from Germany. Only piperacillin-tazobactam showed reliable activity against P. aeruginosa (9% resistant), while resistance to all other agents was >19%. Acinetobacter spp. remained susceptible to only the carbapenems, imipenem and meropenem.

Comparison of the data from Canadian isolates with those from the United States shows some significant differences. This demonstrates the limitations of pooling Canadian and United States data since the differences between the two regions, such as the rate of MRSA, may have some impact on empiric therapy. Data from the NNIS system has previously reported an increasing trend towards resistance within ICUs in the United States [19]. Oxacillin resistance among staphylococci from ICUs in the United States was 52.3% and 84.2% for S. aureus and coagulase-negative species, respectively.

This value is identical to that of S. aureus and very similar to the CNS data reported by the 1999 NNIS system. The NNIS highlighted a 37% increase in MRSA over the period 1994–98 to 1999, but only a 2% increase among CNS strains [4]. Vancomycin resistance in the United States was observed in 4.5% of E. faecalis; however, over 76% E. faecium were vancomycin non-susceptible.

Although streptococci are uncommon ICU pathogens they can be rapidly invasive and possibly fatal unless adequate therapeutic approaches are adopted. S. pneumoniae in the United States has acquired a range of resistance mechanisms with resistance to penicillin and the macrolides, clarithromycin and erythromycin, being common, 20.2% and 25.5%–30.5% respectively. The newer generation cephalosporins, ceftriaxone, cefotaxime and cefepime showed good activity against pneumococci, 3.2%, 6.3% and 4.5% resistant, respectively. Less than 1.0% of isolates were resistant to levofloxacin. These data are similar to other recent reports [20].

For Enterobacteriaceae which account for approximately 30% of all isolates from ICU infections, the incidence of putative ESBLs was low in E. coli, 4.7% but ceftazidime non-susceptibility was higher in K. oxytoca 8.3%,K. pneumoniae 11.5%,S. marcescens 10.3% and M. morganii 26.4%. These data are consistent with other recent reports [21]. Fluoroquinolone resistance was observed in all Enterobacteriaceae tested, in the US for example, resistance rates were as follows, using ciprofloxacin as a marker: E. coli 10.7%, K. oxytoca 5.9%, K. pneumoniae 8.4%, M. morganii 20.7%, P. mirabilis 12.7% and S. marcescens 6.7%. These data show increased fluoroquinolone resistance compared with recent reports [21]. Jones et al. previously reported susceptibility data on ICU pathogens isolated over the period 1998–2001 [22].

Specifically, enteric bacteria showed changes over this time. Fluoroquinolone resistance doubled among E. coli isolates from 3.3–5.5% to 10.8–11.4% [22]. This study showed a generally higher level of activity among third-generation cephalosporins than other reports [23], with ceftriaxone showing <10% resistance rates against most species tested. Piperacillin-tazobactam showed less consistent activity with some species being >14% resistant, e.g. Klebsiella spp.,P. aeruginosa, and Acinetobacter spp. present significant therapeutic challenges in ICUs in the United States. With the exception of cefepime, all other tested antimicrobials demonstrated >12% resistance to P. aeruginosa, many considerably higher. Piperacillin-tazobactam showed the next lowest resistance rate, 14.4%, with all other agents having rates of 17% or higher. Non-susceptibility to ciprofloxacin among P. aeruginosa was 37.2%, higher than in the Neuberger report. Sahm et al. reported a 10% increase in fluoroquinolone resistance among P. aeruginosa in the United States, whereas resistance emerged more slowly with the other classes of antimicrobials tested [12]. Acinetobacter infections continue to present significant therapeutic challenges due to the extensive resistance mechanisms demonstrated by the >25% resistance shown in Table 5. Only imipenem has any reliable activity against Acinetobacter spp. with an 87% susceptibility rate.

There are several implications of these data. It is essential that local surveillance programs be maintained in each country's ICU setting. The local data are vital to the formulary committees as they select appropriate agents to treat infections. There are clear differences among the five countries studied in this report. Although the predominant pathogens are similar, ongoing surveillance is essential to detect the emergence of resistant species. It is clear that certain classes of compounds are losing activity against the ICU pathogens tested. For example, the fluoroquinolones have reduced susceptibility among many Gram-negative species as well as staphylococci; however, the newer class members have enhanced activity against pneumococci. Advanced-generation cephalosporins have variable activity, with ceftriaxone showing consistently good activity against the Enterobacteriaceae and some staphylococci. Ceftazidime has lost potency due to the emergence of ESBL enzymes and also has diminished activity against P. aeruginosa. Piperacillin-tazobactam is generally active against P. aeruginosa in ICUs. The aminoglycoside, gentamicin has shown continued activity against most Enterobacteriaceae in all five countries, and modest activity against S. aureus but not against CNS strains. The gentamicin susceptibility of P. aeruginosa ranged from 44.0% in France to 74.0% in Germany, whereas Acinetobacter spp . showed more variable gentamicin susceptibility varying from 23.3% in Italy to 82.0% in Germany. These local data should be considered when treating infections in the ICU.

Use of agents with anti-pseudomonal activity such as cefepime, piperacillin-tazobactam or the carbapenems should preferably be reserved for patient types or infections where this pathogen is present or risk factors exist, as per the ATS Community acquired-pneumonia guidelines [24]. A combination of a third-generation cephalosporin such as ceftriaxone with vancomycin may be appropriate for bloodstream infections based upon the NNIS etiology data from 1992–1999.

Conclusions

The current study confirmed the emergence of fluoroquinolone resistance among various Gram-negative species and staphylococci, which may be increasing due to the heightened use of these drugs; however the reported ESBL rates among Enterobacteriaceae was lower than noted in other studies and appeared to be stable. The prevalence of MRSA, perhaps the most significant resistant hospital pathogen, varied among the five countries and appeared to be increasing. Parenteral cephalosporins such as ceftriaxone and cefotaxime remained quite active against Enterobacteriaceae. Up-to-date susceptibility data should be made available as rapidly as possible to physicians so that appropriate targeted empirical therapy can be instituted, this approach can assist in maintaining the activity of the current antimicrobials. While local surveillance studies remain crucial, national surveillance studies such as this can provide an invaluable data source to provide guidance in formulary decision-making.

Authors Contributions

MJ conceived the study, provided data interpretation and drafted the manuscript. DD analyzed the study data; JK and DS provided expert microbiological analysis and interpretation of study data; RW provided clinical expertise in interpretation of data and drafting manuscript. All authors read and approved the final manuscript.