Antibiotic Sensitivity Pattern of Blood Isolates of Acinetobacter Species in a Tertiary Care Hospital : A Retrospective Analysis

Problem statement: Multi-drug resistant Acinetobacter bacterium is one of the major causes of sepsis in ICUs in tertiary care hospitals n India. In this report we describe the antibioti c sensitivity patterns of Acinetobacter species isolated from blood over a one year period at a tertiary care hospital. Approach: We retrospectively analyzed the sensitivity patter n of Acinetobacter species isolated from blood during the period 1/6/2010 to 3 1/5/2011. Isolation and identification were performed using the best alert system and VITEK2 re spectively. Sensitivities were determined by Kirby Bauer disc diffusion and broth dilution using VITEK2 -AST cards. Results: The total number of Acinetobacter species isolated during the study period was 72, o ut of which 57 (79%) were A. baumanii, 7 (9.7%) were A. Iwofii and 3 (5.2%) were A. Junii. One each from A. calcoaceticus, A. ursingii and A. denitrificans were isolated. All of the baumanii isolates were sensitive to polymyxin B and 61.4% were sensitive to tigecycline. Only 25% o f the isolates in baumanii group were sensitive to meropenem and imipenem. In the non-baumanii group however, 73% were sensitive to carbapenems. Conclusion: There is a very high incidence of resistance to mos t antibiotics, including carbapenems. All of the Acinetobacter isolates tested are sensitive to polymyxin B. Tige cycline is the only other drug with reasonable susceptibilities, but this drug is not recommended for primary bacteriemias. If Acinetobacter sepsis is suspected, empiric therapy with polymyxi ns, followed by de-escalation after sensitivity results are back, is advisable.


INTRODUCTION
Members of the genus Acinetobacter are ubiquitous, free living, aerobic, Gram negative coccobacilli that prefer a moist environment and can be easily obtained from soil, water, food and sewage (Dougari 2011).
They are usually considered to be opportunistic pathogens and cause nosocomial infections in hospitalized patients like bacteremia, pneumonia, meningitis and Urinary Tract Infection (UTI) (Towner, 1997;Bergogne-Berezin et al., 1996).
Antimicrobial resistance among nosocomial isolates of Acinetobacter complicates therapy and adversely affects clinical outcomes and treatment costs (Brusselaers et al., 2011;Harris et al., 1999).The presence of resistance to most antibiotic classes requires the use of older and more toxic drugs like colistin for tackling Multi-Drug Resistant (MDR) strains (Fritsche et al., 2005).
We analyzed the resistance pattern of Acinetobacter species grown in blood culture in patients during a one year period.

MATERIALS AND METHODS
All isolates of Acinetobacter from blood over a period of 1 year (1/6/2010 to 31/5/2011) were included in this study.Isolation and identification was performed using the Bac Talert and the VITEK 2 machine respectively.Antibiotic sensitivities were determined by Kirby Bauer disc diffusion and broth dilution using VITEK2 and AST cards and interpreted according to CLSI criteria.

RESULTS
The total number of Acinetobacter species isolated during the study period was 72, out of which 57 (79%) were A. baumanii, 7 (9.7%) were A. Iwofii and 3 (5.2%)were A. junii.One each from A. calcoaceticus, A. haemolyticus, A. ursingii, A. denitrificans were isolated.One isolate did not belong to any of the above species.
All the isolates in the Acinetobacter baumanii group were sensitive to polymyxin B (Table 1).61.4% were sensitive to tigecycline.Sensitivities of imipenem, meropenem and doripenem were 24.55, 24.5 and 22.5% respectively.Only 12.5% of the isolates were sensitive to cefuroxime whereas for cefotaxime and ceftriaxone it was 12.2 and 10.5% respectively.Cefepime and ceftazidime had marginally better sensitivities when compared with other cephalosporins (28% each).Cefaperazone + sulbactam combination had better sensitivities (33%) when compared to piperacillin + tazobactam (21%).
The 15 isolates of non Baumanii group differed significantly from the Baumanii group, which had much greater sensitivities to all classes (Table 2).

DISCUSSION
Acinetobacter has become one of the most important causes of nosocomial infections (Gerner-Smidt, 1995;Gulati et al., 1999) and causes considerable mortality as it has acquired many antibiotic resistance genes including the novel carbapenemases.It is an opportunistic pathogen associated with a wide spectrum of infections including nosocomial pneumonia, meningitis, endocarditis, skin and soft tissue infections, urinary tract infections, conjunctivitis, burn wound infections and bacteremia (Bergogne-Berezin and Towner, 1996).The common form of resistance to carbapenems is mediated by lack of drug penetration (i.e., porin mutations and efflux pumps) and/or carbapenem hydrolyzing beta-lactamase enzymes including the Metallo-Betalactamases (MBL).Acquired MBLs are encoded mobile gene cassettes of organism and such strains are often resistant to different groups of antimicrobial agents with transferable properties to various types of bacteria (Pitout et al., 2005).
Resistance rates to carbapenems vary significantly depending on the geographical region.In Greece, the proportion of imipenem-resistant A. baumanii isolates from patients hospitalized between 1996 and 2007 in tertiary care hospitals in several regions of the country rose from no resistance to 85% (ICUs), 60% (medical wards) and 59% (surgical wards) GSSAR.Bloodstream isolates from the same dataset exhibited even higher resistance rates.The prevalence of imipenem resistance in A. baumanii isolated from a burns unit of USA was found to be as high as 87% (2007) (Trottier et al., 2007).
Initial Indian studies in the 21st century showed that Acinetobacter species were fairly sensitive.For instance, Suri et al. (2000) demonstrated Acinetobacter in patients from a neurosurgical unit and it was sensitive to ciprofloxacin, amikacin cefotaxim and ceftriaxone.Singh et al. (2002) showed Acinetobacter which was sensitive to amikacin.Prashanth and Badrinath (2004) from JIPMER Pondichery isolated Acinetobacter which was sensitive to amikacin and ceftazidime.Isolates were resistant to ciprofloxacin and cefotaxime (Prashanth and Badrinath, 2004).Gladstone et al. (2005) from Vellore reported a prevalence of 14% carbapenem-resistant Acinetobacter spp., isolated from tracheal aspirates.Banerjee et al. (2005) isolated Acinetobacter from different body fluids which has good sensitivities for gentamycin.Prashanth and Badrinath (2006) showed gradually increasing resistance of Acinetobacter.Gaur et al. (2008) noted resistance to meropenem in 6.4% of Acinetobacter species.As recently as in 2010, one study from Ahmedabad showed few were carbapenem resistant (Patel et al., 2010).
However there are now several Indian studies showing an increased prevalence of MDR Acinetobacter.Our study shows that 75% of our isolates were carbapenem resistant.This is concordant with recent reports from elsewhere in India.In 2009 a study from Rohtak showed that the resistance of Acinetobacter to meropenem had increased to 25.6% (Goel et al., 2009).In the same study the resistance to amikacin was 87.2% and ciprofloxacin was 89.7%.In Delhi, India (2006), the prevalence of carbapenem resistance in Acinetobacter spp.isolated from different clinical samples was found to be almost 35%.Sinha et al. (2007), but the latest studies show resistance to carbapenem is seen in up to 89% of isolates (Jaggi et al., 2011).In our study the level of carbapenem resistance was very high in the Baumanii group.Karthika et al. (2009) in their study showed the presence of bla IMP1 carbapenemase genes in South Indian population.Though bla VIM-2 is the most common carbapenemases seen in other parts of world (Yum et al., 2002;Poirel et al., 2000;Walsh et al., 2005), it was surprisingly absent in their study, though bla IMP1 gene was seen in 42% of isolates (Karthika et al., 2009).Our study showed 100% sensitivity to polymyxin.Tigecycline too retained activity against MDR isolates, although it is not recommended for primary bacteremias.However there have been reports of polymyxin resistant Acinetobacter from Greece, Slovakia and other parts of Europe (Gales et al., 2006;Souli et al., 2006).Polymyxin resistance has been reported from South Korea and the rates of resistance is alarming, 18.1 and 27.9% for polymyxin B and colistin respectively (Ko et al., 2007).Recently there has been an alarming study from Chandigarh where 3.5% of all strains and 16% of carbapenem resistant strains were resistant to polymyxin B and tigecycline (Taneja et al., 2011).
The antibiotic sensitivity pattern of non Baumanii group differed significantly from Baumanii group.Most of the isolates were sensitive to carbapenems and BL+ BLI combinations (73%).
The strength of our study is that only blood isolates were analysed, as samples from other sites may represent colonization and may not reuire therapy.The polymyxin group remains the only option as an empirical therapy if Acinetobacter bacteremia is suspected, as it showed 100% sensitivities to MDR Acinetobacter.Though tigecycline showed good antibiotic sensitivity in our study, it is not recommended for primary bacteremia due to low blood levels resulting inclinical failures.

CONCLUSION
There was a very high incidence of resistance to most antibiotic classes, including carbapenems, in Acinetobacter blood isolates in our center.All of the resistant isolates were however sensitive to polymyxin B. Tigecycline was the only other drug with reasonable susceptibilities.If Acinetobacter bacteremia is suspected, empiric therapy with the polymyxin group, followed by de-escalation after sensitivity results are back, is advisable.