Etiological profile and antimicrobial susceptibility pattern of blood culture isolates for bloodstream infection

INTRODUCTION

Bloodstream infections (BSIs) represent a significant global burden, contributing substantially to both morbidity and mortality. They are identified by the presence of a positive blood culture in a patient exhibiting systemic signs of infection and originate either primarily or result from an established infection at another body site.^[1] In critically ill patients, each hour of delay between symptom onset and the initiation of antibiotic therapy raises the mortality rate by 9%. Therefore, prompt identification of the causative agents responsible for these infections, along with their antimicrobial susceptibility profiles, is crucial for effective patient management.

Patients admitted in critical care units are particularly vulnerable to colonization by highly resistant pathogens due to factors such as overuse of broad-spectrum antibiotics, compromised immune system, presence of indwelling devices, and need for invasive procedures. An effective Antimicrobial Stewardship Program (AMSP) ensuring appropriate use of antibiotics is vital for effectively managing these infections. Baseline data about the microorganisms responsible for such infections and their antimicrobial susceptibility profile protect against the irrational use of antibiotics and aid in the prevention of antimicrobial resistance (AMR). Therefore, this study was conducted to identify the organisms responsible for BSIs in our setup and to assess the antimicrobial susceptibility patterns of the isolated strains.

MATERIALS AND METHODS

RESULTS

Ten thousand four hundred forty-nine blood samples were received in the microbiology laboratory during the study period. Among these, 1378 samples (13.2%) showed growth of pathogenic organisms, comprising 1,228 bacterial isolates (89.1%) and 150 Candida isolates (10.9%). A male-to-female ratio of 2.27:1 was observed, and the highest positivity rate was found in the 46–60 age group. The mean age of patients with positive blood cultures was 51 years, ranging from 18 to 91 years. The majority of samples were received from intensive care unit (ICU) patients (99.4%).

Figure 1 summarizes the results of the study, showing that Gram-negative bacilli were the most common isolates (810/1,378, 58.8%). The most common organism recovered was Klebsiella pneumoniae (309/1,378, 22.4%), followed by CoNS (240/1,378, 17.4%). Candida tropicalis was the most common fungal isolate (43/150, 28.7%).

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Figure 1:

Spectrum of most common microorganisms isolated from blood culture during the study period.

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Gram-positive cocci showed the highest susceptibility to linezolid, vancomycin, and daptomycin [Table 1].

Tables 2 and 3 present the antimicrobial and antifungal susceptibility data of the most common Gram-negative and fungal isolates, respectively.

A high rate of AMR was noted, including (26/46, 56.5%) methicillin-resistant Staphylococcus aureus (MRSA), (9/125, 7.2%) vancomycin-resistant enterococci (VRE), and (269/536, 50.2%) carbapenem-resistant enterobacterales (CRE).

DISCUSSION

The current study aimed to identify the spectrum of microorganisms responsible for BSIs and to analyze their antimicrobial susceptibility profiles. In the present study, blood culture positivity was detected in 13.2% of the samples, with the majority of culture-positive cases (99.4%) found among in-patients. This rate is consistent with findings from several studies conducted in India^[2,3] and abroad.^[4,5] Higher culture positivity was observed by Pal and Sujatha^[6] and Vasudeva et al.^[7] Differences in blood culture results can be linked to factors such as blood specimen volume, timing of collection, culture techniques, and variations in the epidemiology of causative agents. This study reveals a higher prevalence of BSI in males, with a male-to-female ratio of 2.27:1 and the highest positivity rate observed in the 46–60 age group. Several other studies have also documented similar results.^[8]

Gram-negative BSIs were more common, a trend consistent with findings from other studies. Among Gram-negative pathogens, enterobacterales accounted for the maximum cases (66.2%) with a predominance of Klebsiella spp. (38.1%), followed by Escherichia coli (23.6%), [Figure 1] which corroborates with a study done by Banik et al.^[1]

The antibiotic susceptibility pattern of common Gram-negative pathogens is summarized in Table 2. They exhibited low susceptibility to fluoroquinolones, aminoglycosides, and third-generation cephalosporins, except for E. coli, which showed moderate susceptibility to fluoroquinolones. The CRE prevalence in our study is 50.2%, which is higher than the prevalence reported by Modi et al. (29.07%).^[9] Similarly, a high prevalence was observed in the study by Bajaj et al.^[10] with 67.3%. These results highlight a significant rise in carbapenem resistance, likely driven by the overuse and misuse of carbapenems.

Non-fermenting Gram-negative bacteria (NFGNB) accounted for 33.8% of the isolates in our study, among which Acinetobacter spp. was the most common (50%), followed by Pseudomonas spp. (18.1%), Burkholderia cepacia (12%) and Stenotrophomonas maltophilia (7.2%) [Figure 1]. These findings are consistent with those of Manjunath et al.^[11] who reported a similar isolation rate of 28.3% of NFGNB. Our findings indicated varying drug susceptibilities for B. cepacia, with trimethoprim-sulfamethoxazole and ceftazidime being 100% effective options. Similar resistance patterns were seen in studies done by Kady et al.^[12] and Omar et al.^[13] Drugs of choice for S. maltophilia include trimethoprimsulfamethoxazole, fluoroquinolones, and minocycline. Our findings showed 70–100% susceptibility to these drugs, consistent with the study done by Bacchani et al.^[14] [Table 2]

Among 418 Gram-positive cocci, CoNS predominated (57.4%), followed by Enterococcus spp. (29.9%) and S. aureus (11%) [Figure 1]. Staphylococcus hemolyticus was the most common CoNS (47.9%) isolated, which is concordant with studies done by Gopalakrishnan and Sureshkumar^[15] and Edwards et al.^[16] Isolation from paired blood culture and correlation with clinical signs and biomarkers were carefully ascertained before reporting CoNS as a true pathogen. The high prevalence of CoNS in our study can be linked to our position as a tertiary care hospital that caters to a substantial population of immunocompromised patients and those with comorbidities.

MRSA prevalence in our BSI isolates is 56.5%. They are known to vary from 25% in the western part of the country to 50% in the southern regions.^[17] Chaturvedi et al.^[18] reported similar findings, highlighting the high prevalence rates of MRSA. In contrast, research conducted by Banik et al.^[1] and Vasudeva et al.^[7] indicated lower rates of MRSA. Nonetheless, MRSA remains a global threat to public health. Vancomycin is the primary treatment choice for MRSA infections. However, the emergence of vancomycin-resistant S. aureus also poses a growing concern. In the present study, S. aureus showed 100% susceptibility to vancomycin, daptomycin, and linezolid, consistent with other contemporary studies^[1,3,7,18] [Table 1].

Among 224 erythromycin-resistant Staphylococcal isolates, 66% exhibited inducible clindamycin resistance (ICR), significantly higher than our previous study by Shrigaur et al. (30.6%).^[19] Staphylococcus spp. also exhibited moderate susceptibility to gentamicin (82.6%), cotrimoxazole (80.4%), and tetracycline (89.1%), while showing high resistance to ciprofloxacin (91.3%), consistent with findings by Gade and Qazi^[20] [Table 1].

The prevalence of Enterococcus spp. among Gram-positive bacteria was 29.9%, with Enterococcus faecium and Enterococcus faecalis comprising 85% (107/125) and 12.8% (16/125), respectively. High-level gentamycin resistance was observed in 81.6% of isolates. Susceptibility rates observed for linezolid, vancomycin, and teicoplanin were 96.8%, 92.8%, and 93.6%, respectively [Table 1]. VRE prevalence in the current study was found to be 7.2%. Over the past decade, the prevalence of VRE in India has risen, with reported rates ranging from 1% to 8.7%.^[21]

Candida spp. accounted for 10.9% of BSIs in our setup, with C. tropicalis being the most common (28.7%) [Figure 1]. Globally, non-albicans Candida represents 60% of candidemia cases^[22,23] and our study similarly reported 58%. Known risk factors for candidemia include prolonged health care and antibiotic exposure, ICU admission, and immunocompromised states.^[24] Our isolates showed high susceptibility to echinocandins, the preferred treatment for candidemia [Table 3], consistent with findings in our previous studies.^[25]

This study has a few limitations inherent to its retrospective design, with poor control over confounding factors. Furthermore, this is a single-center study, therefore data may not be generalizable. Yet, it provides vital insight into the current epidemiology of BSIs and their antimicrobial susceptibility profile. This data shall help in developing empirical therapy guidelines and providing a baseline for timely and effective management of such infections, thereby reinforcing the principles of antimicrobial stewardship.

CONCLUSIONS

BSIs are an important cause of morbidity and mortality among hospitalized patients. Gram-negative organisms have been found to be the most common cause, with K. pneumoniae being the most common isolate. A high level of AMR is observed among the pathogens, which necessitates the formulation and strict compliance of effective AMSP.