4. Discussion
Tigecycline is licensed in Italy to treat complicated intra-abdominal and skin and soft tissue infections. Good clinical results for tigecycline have been demonstrated previously: for example, Bassetti
et al. [
11] showed a 76.5% (13/17) success rate for tigecycline in the treatment of complicated skin and soft tissue infections and an 82.8% (72/87) success rate against peritonitis. This clinical success is reflected by high tigecycline susceptibility for most pathogens over the course of the T.E.S.T. study between 2004 and 2011: Gram-negative pathogens showed >94% tigecycline susceptibility while Gram-positives were >98% susceptible in the present report.
The two Gram-negative organisms in this study for which tigecycline breakpoints were not available were
A. baumannii and
P. aeruginosa. Against
A. baumannii, tigecycline had the lowest overall MIC
90 (2 mg/L) of all T.E.S.T. agents in Italy. A previously published Italian surveillance study comprising nine hospitals also determined an MIC
90 of 2 mg/L for tigecycline against
A. baumannii isolates collected between 2003 and 2004 [
12]. As in the current analysis, the one-year study found that
A. baumannii isolates had low (≤51%) susceptibility to amikacin, cefepime, ceftazidime, meropenem and piperacillin-tazobactam. The proportion of
A. baumannii resistant to piperacillin-tazobactam in the 2003–2004 study (44%) was lower than to the overall value in this T.E.S.T. paper (63.3%; 2004–2011). However, the 2003–2004 frequency of imipenem-resistant isolates (50%) was around 36% higher than T.E.S.T. (13.7%; 2004–2006). Both studies used CLSI methodologies to determine imipenem susceptibility, but there were 27 countrywide centres collecting isolates for T.E.S.T., compared with nine centres in the central-south region of Italy for the 2003–2004 study. The high incidence of imipenem resistance in the one-year study could, therefore, be due to a localised outbreak of imipenem-resistant
A. baumannii infections in this region of Italy during 2003–2004.
A. baumannii gained resistance to most antimicrobials on the panel over the course of the T.E.S.T. study in Italy as described in this report, and the greatest increase in resistance was to piperacillin-tazobactam (49.1% from 2004 to 2011). There were also >40% increases in resistance to amikacin and levofloxacin.
Epidemiological studies of
A. baumannii in Italy have focussed mostly on carbapenem-resistant or MDR isolates. Lambiase
et al. [
13] examined 567
A. baumannii isolates from an ICU in Naples between 2007 and 2010, and found that all isolates were MDR, including carbapenems; these isolates were clonal in nature, all possessing the
blaOXA-51-like and
blaOXA-58-like genes. D’Arezzo
et al. [
14] reported a high (60.5%) prevalence of elevated (MIC≥128 mg/L) resistance to imipenem among 111
A. baumannii isolates, associated with
blaOXA-58-like (22.8%) or
blaOXA-51-like (71.1%) genes. Most of these isolates (95.6%) were related to international clonal lineage II. In a study of six hospitals in Florence, Donnarumma
et al. [
15] showed three main clonal groups of
A. baumannii, A1, A2 and A3; A1 was genetically related to the European EU II clone. All isolates possesses the
blaOXA-51-like gene, and 65% of these isolates were resistant to imipenem. As reported globally [
16], clonal lineages appear to predominate among
A. baumannii isolates in Italy, although there may be variation between different geographical areas.
In the 2003–2004 Italian study by Mezzatesta
et al. [
12], 49.5% of the 107
A. baumannii isolates tested were resistant to three antimicrobial classes (specifically fluoroquinolones, ceftazidime, and aminoglycosides or imipenem). This was only slightly lower than the proportion of MDR
A. baumannii collected over the T.E.S.T. surveillance period in Italy (60%). In a separate study of seven centres in Rome carried out from 2004 to 2005, antimicrobial susceptibility was determined for 22 MDR
A. baumannii patient isolates [
17]. Of these 22 isolates, 21 (95.5%) were resistant to levofloxacin, 19 (86.4%) were resistant to piperacillin-tazobactam, and 14 (63.6%) were resistant to amikacin. The MDR
A. baumannii isolates collected in T.E.S.T. were highly resistant to levofloxacin (98.1%), piperacillin-tazobactam (93.7%) and amikacin (87.5%) over all years of surveillance. In the seven-centre study by Principe
et al. [
17], three MDR
A. baumannii isolates (13.6%) were resistant to tigecycline (using the FDA Enterobacteriaceae breakpoints for tigecycline [S, ≤2 mg/L; R, ≥8 mg/L]). A single Italian hospital also determined drug resistance among 50 MDR
A. baumannii isolates collected between 2008 and 2009 [
18]. Only 4% of these
A. baumannii isolates were resistant to tigecycline (S, ≤2 mg/L; R, ≥8 mg/L). All isolates, however, were 100% resistant to imipenem, levofloxacin and piperacillin-tazobactam, and more than 90% resistant to amikacin, cefepime and ceftazidime. In this T.E.S.T. study, MDR
A. baumannii isolates were highly resistant (>79%) to the same antimicrobials, excluding imipenem (38.8%) and minocycline (5.1%).
Compared with
A. baumannii, a lower overall proportion of MDR
P. aeruginosa (19.1%) was recorded in Italy during T.E.S.T. In 2010, a similar percentage of invasive
P. aeruginosa isolates from Italy (20.8%) was reported to have resistance to three or more antibiotic classes among aminoglycosides, carbapenems, ceftazidime, fluoroquinolones, and piperacillin-tazobactam [
19]. The individual resistance values for ceftazidime and piperacillin-tazobactam against these invasive isolates were 21.2% and 17.7%, respectively, slightly lower than the 2010 T.E.S.T. results in Italy (32.9% resistance for both agents).
The highest proportion of resistant Gram-negative pathogens in the current Italian study was observed among ESBL-producing
E. coli (24.8% from 2004–2011). This is a dramatic increase compared to the 10.8% occurrence of ESBL-positive
E. coli isolates nationally in 1999 [
20]. An Italian single-hospital surveillance report from 2004 to 2007 identified 23.5% of
E. coli as ESBL producers [
21]. A more recent (2009–2010) study of another Italian hospital found that the most frequently observed multidrug-resistant pathogen was ESBL-producing
E. coli (18.6% of all multidrug-resistant isolates) [
22]. These data are in line with the findings from the current T.E.S.T. manuscript.
ESBL production has been linked with third-generation cephalosporin resistance [
23]. The T.E.S.T. data showed that the resistance of
E. coli to ceftriaxone in Italy increased more than two-fold between 2007 and 2008, during which time the prevalence of ESBL-producing
E. coli isolates almost doubled. In 2010, the ECDC annual report found that in one Italian centre, all 23 of the invasive
E. coli isolates resistant to third-generation cephalosporins were ESBL-producers [
19]. In a 2007–2008 single hospital study of 13 countries, including Italy, higher mortality rates and longer hospital stays were associated with third-generation cephalosporin-resistant
E. coli bloodstream infections [
24]. These findings suggest that
E. coli resistance in Italy is increasing, possibly due to the spread of ESBL-positive strains.
High proportions of ESBL-producing
K. pneumoniae were also measured during T.E.S.T. (24.1% over all years). ESBL production has been associated with reduced carbapenem susceptibility in
K. pneumoniae due to a loss of bacterial membrane permeability in some ESBL-producing isolates [
25]. The recent ECDC report noted an increase in carbapenem-resistant
K. pneumoniae, from 1% in 2006 to 27% in 2011 [
1]. Another recent Italian paper has also highlighted a rise in carbapenem (imipenem and/or meropenem) non-susceptibility (intermediate plus resistant isolates) in
K. pneumoniae, from 2.2% to 19.4% between 2009 and 2012, respectively [
26]. These results are supported by the current T.E.S.T. study, in which
K. pneumoniae resistance to meropenem increased in Italy from 1.4% in 2006 to 14.4% in 2011.
Although Gram-positive organisms were wholly susceptible to tigecycline, certain pathogens had low susceptibility to other antimicrobials during this T.E.S.T. study.
E. faecium, for example, showed 79.8% resistance to ampicillin over all years of Italian surveillance. Similarly, an earlier report documented 70% ampicillin resistance among 913
E. faecium isolates from 20 Italian centres between 1993 and 1995 [
27]. The more recent report from a single Italian teaching hospital by Manfredi and Nanetti [
21] also found that ampicillin had limited activity against
E. faecium (7.5%-18.5% susceptibility in 175 isolates from 2004–2007). Vancomycin resistance in
E. faecium, however, appears to be declining in Italy, as the ECDC reported 21% vancomycin-resistant isolates in 2004 but 4% in 2010 [
19]. This T.E.S.T. study showed similar results in 2004 (18.2% vancomycin resistance), decreasing to 13.3% resistance in 2010, before a further decline to 0% in 2011. This reduction in vancomycin-resistant
E. faecium may be related to increased use of infection-control strategies in hospitals, which have been shown to reduce the incidence of vancomycin-resistant enterococci [
28].
The current T.E.S.T. data showed that 33.8% of
S. aureus isolates in Italy were resistant to methicillin, increasing in prevalence between 2007 (27.8%) and 2011 (32.9%) (although an overall decrease was noted compared to 2004 [45.1%]). Similarly, the ECDC reported a small increase in the proportion of MRSA between 2007 and 2010, from 33% to 37% [
19]. The distribution of MRSA across Italy is complicated. In Torino, characterization of 90 MRSA isolates revealed that most belonged to SCCmec types I and II [
29], while in the Emilia-Romagna region, 63% of MRSA clones showed spa-types t008 or t041 [
30]. An examination of 10 Panton-Valentine leukocidin-positive MRSA isolates from Bolzano province revealed a heterogeneous sample, with eight different ST clonal types identified [
31]. Italy thus possesses a polyclonal population of MRSA, with different clonal types occurring in separate regions in differing proportions. Also, there has been a blurring of the distinction between community-acquired and hospital- acquired MRSA: one recent report has shown a high prevalence of the USA-300 clone in central Italy among both community- and hospital-acquired isolates of MRSA [
32].
Two key antimicrobials in the treatment of infections caused by MRSA are vancomycin and linezolid. There have been numerous reports of increasing non-susceptibility to vancomycin among MRSA isolates in recent years [
33]. However, there is no evidence of this in the current study in Italy: vancomycin retains 100% activity against
S. aureus (and MRSA) through the T.E.S.T. study in 2004–2011. Vancomycin “MIC creep” appears to be a regional occurrence and not yet a generalized trend; medical institutions should thus regularly monitor local vancomycin susceptibility among MRSA isolates [
34]. Similarly, linezolid has retained its normal good activity against
S. aureus (and MRSA) in the current study, with 100% susceptibility reported over all study years. Linezolid has previously been shown to be active against vancomycin-intermediate isolates of MRSA collected in Italy, so is an important clinical tool in the fight against resistant infections [
35].
Some important differences are observed when resistance rates in Italy are compared to global rates, as reported in Pfizer’s online T.E.S.T. database [
36]. In general, resistance levels to β-lactams (particularly ceftriaxone) and levofloxacin appear to be higher in Italy than globally.
A. baumannii resistance to most antimicrobial agents is higher in Italy by approximately 15%, with the exceptions of amikacin and levofloxacin (resistance in Italy is > 20% higher) as well as imipenem and minocycline (resistance is similar).
E. coli resistance is also high in Italy: ampicillin and cefepime resistance are around 7% higher, while ceftriaxone and levofloxacin resistance are approximately 12% higher than globally. Among
Enterobacter spp., resistance was roughly 10% higher in Italy to ceftriaxone, levofloxacin and piperacillin-tazobactam. Amoxicillin-clavulanate, cefepime, ceftriaxone, levofloxacin and piperacillin-tazobactam resistance among Italian isolates of
K. pneumoniae are approximately 6–8% higher than isolates globally. Resistance is roughly 6–10% higher among isolates of
P. aeruginosa from Italy to all antimicrobial agents (excluding amikacin, to which resistance is the same in Italy as globally). Among
S. pneumoniae, macrolide, minocycline and clindamycin resistance are about 10%, 12% and 14% higher in Italy, respectively, while penicillin resistance is approximately 7% lower than the global average. Implementation of and strict adherence to resistance control measures, such as ongoing resistance surveillance, improved hand hygiene/increased glove use and/or the use of antimicrobial stewardship programs [
37,
38], would almost certainly help to reduce the high levels of resistance observed in Italy.
The T.E.S.T. study, like all surveillance studies, suffers from inherent limitations. Although several centres participated in the T.E.S.T. study in Italy over 2004–2011, some contributed isolates over several years while others participated in a few years or even just one, causing fluctuations in isolate contribution both geographically and over time. Thus, regional variations in resistance in a given study year may have had a disproportionate influence on apparent national resistance levels.
Over all T.E.S.T. years, rates of β-lactamase-producing H. influenzae, ESBL-producing K. oxytoca, vancomycin-resistant E. faecalis and PRSP were ≤11.4% in Italy. Another encouraging result of this study was the identification of no vancomycin-resistant Enterococci in Italy in 2011. The above findings may indicate that these drug-resistant organisms are becoming less prevalent in Italian hospitals, and therefore, less of a threat to the welfare of patients. Other pathogens in Italy, including E. coli and K. pneumoniae, have shown increased resistance in recent years, due largely to the spread of ESBL-positive strains. Thus, these organisms must continue to be monitored for further changes in susceptibility in the future. The results of surveillance studies such as T.E.S.T. help members of the healthcare industry to monitor rates of in vitro susceptibility among important pathogens to widely used antimicrobial agents, both globally and regionally.
