Introduction

Bloodstream infections (BSIs) are a significant contributor to morbidity and mortality among critically ill patients and continue to impose a considerable burden on healthcare systems. The risk is particularly heightened in intensive care units (ICUs), where factors such as compromised host defenses, frequent use of invasive medical devices, prolonged hospital stays, and extensive exposure to broad-spectrum antimicrobials predispose patients to systemic infections [1,2]. Collectively, these conditions create a highly vulnerable environment that facilitates microbial invasion and dissemination into the bloodstream.

The microbial epidemiology of BSIs in ICU settings is highly variable and often reflects local patient characteristics, antimicrobial usage patterns, and infection control practices. In recent years, Gram-negative bacilli have emerged as the predominant pathogens in many critical care settings, especially species such as Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. These organisms are frequently associated with multidrug resistance, significantly complicating therapeutic management. At the same time, Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA) and Enterococcus spp., continue to contribute substantially to the burden of bloodstream infections. Additionally, fungal pathogens—particularly Candida species—have gained increasing importance in patients with prolonged ICU stays and multiple comorbidities [3–5].

A key challenge in the management of BSIs is the rapid and ongoing rise in antimicrobial resistance (AMR). The empirical use of broad-spectrum antibiotics, although essential for early sepsis management, exerts considerable selective pressure that facilitates the emergence and spread of resistant organisms. This includes extended-spectrum β-lactamase (ESBL)-producing Enterobacterales, carbapenem-resistant Gram-negative bacilli, and vancomycin-resistant enterococci (VRE), all of which are associated with limited therapeutic options and poorer clinical outcomes [6–8].

Device-associated infections, particularly those related to central venous catheters, represent a significant proportion of ICU-acquired BSIs. The ability of microorganisms to form biofilms on intravascular devices enhances their persistence, reduces susceptibility to antimicrobial agents, and protects them from host immune responses, thereby contributing to treatment failure and recurrence [9,10].

Given that definitive microbiological identification and susceptibility results are typically available only after 48–72 hours, the initial management of BSIs relies heavily on empirical antimicrobial therapy. The appropriateness of this therapy is critically dependent on up-to-date knowledge of local pathogen distribution and resistance patterns. Therefore, continuous surveillance and periodic analysis of institutional antibiograms are essential for guiding empirical treatment strategies and strengthening antimicrobial stewardship efforts [11,12].

In resource-constrained settings such as India, additional challenges—including high patient load, inconsistent infection control measures, and unregulated antibiotic usage—further accelerate the emergence and dissemination of resistant pathogens. Consequently, generating institution-specific data on the microbial profile and antimicrobial susceptibility patterns of BSIs is crucial for optimizing therapeutic decision-making and improving patient outcomes.

In this context, the present study was conducted to evaluate the microbial spectrum and antimicrobial resistance patterns of bloodstream infections in a multidisciplinary intensive care unit at a tertiary care hospital in North East Delhi, with the objective of providing locally relevant evidence to inform empirical therapy and infection control practices.

Materials and Methods

Study design, setting, and duration

This retrospective cross-sectional study was conducted in the Department of Microbiology, University College of Medical Sciences (UCMS) and Guru Teg Bahadur Hospital (GTBH), New Delhi. The study included blood culture–positive patients admitted to the multidisciplinary intensive care unit (MICU) of a tertiary care hospital over a one-year period from January 2025 to December 2025.

All blood culture samples received in the microbiology laboratory from patients admitted to the multidisciplinary ICU during the study period were processed as part of routine diagnostic workflow. Samples were inoculated into automated blood culture bottles and incubated in the BACT/ALERT system (bioMérieux, France) for continuous monitoring.

Following a positive signal, a direct Gram stain was performed from the positive broth. The specimen was then sub-cultured onto 5% sheep blood agar and MacConkey agar. Plates were incubated at 37°C for 24 hours under appropriate atmospheric conditions. Additional media, including Sabouraud dextrose agar was used where fungal infection was suspected.

Microbiological identification

Microbial isolates were identified on the basis of colony morphology, Gram staining characteristics, and standard biochemical tests. Identification was further confirmed using catalase, coagulase, oxidase, and motility testing, along with carbohydrate fermentation profiles. Members of the family Enterobacteriaceae were identified and differentiated using a battery of standard biochemical tests.

Antimicrobial susceptibility testing

Antimicrobial susceptibility testing was performed using the modified Kirby–Bauer disc diffusion method on Mueller–Hinton agar, interpreted according to Clinical and Laboratory Standards Institute (CLSI) guidelines. Quality control was ensured using standard ATCC strains: Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), and Pseudomonas aeruginosa (ATCC 27823). In fungal identification, Yeast isolates were cultured on Sabouraud dextrose agar and CHROM agar. Identification was performed using germ tube test, cornmeal agar morphology, and carbohydrate fermentation profiles.

Data were analysed using SPSS version 27.0. Categorical variables were expressed as frequencies and percentages. Comparisons between categorical variables were performed using the Fisher’s exact test. A p-value < 0.05 was considered statistically significant.

Results

A total of 160 bloodstream infection (BSI) cases were analysed during the study period. The highest proportion of patients belonged to the 21–30 years age group (25%), followed by 31–40 years and 51–60 years. A female predominance (55%) was observed [Table 1].

As illustrated in Figure 1, Gram-negative bacilli (GNB) constituted the predominant group of isolates, followed by Gram-positive cocci (GPC). The detailed microbial spectrum [Figure 2] demonstrated that among Gram-negative organisms (n = 134), Acinetobacter baumannii was the most frequently isolated pathogen (44%), followed by Enterobacter spp. (26.9%), Pseudomonas aeruginosa (13.4%), Klebsiella pneumoniae (12.7%), and Citrobacter spp. (3%). Among Gram-positive isolates (n = 21), Staphylococcus aureus (n = 14) predominated over Enterococcus spp. (n = 7).

The antimicrobial susceptibility profile of Gram-negative isolates [Table 2] revealed widespread resistance to cephalosporins and fluoroquinolones. Carbapenems and aminoglycosides retained comparatively better activity across most isolates. However, Acinetobacter baumannii exhibited marked multidrug resistance, with carbapenem susceptibility limited to approximately 20%, underscoring its critical role as a resistant nosocomial pathogen.

Among Gram-positive organisms [Table 3], Staphylococcus aureus demonstrated 53.8% susceptibility to cefoxitin, indicating a substantial burden of methicillin resistance. Moderate susceptibility was observed to clindamycin (53.8%), while erythromycin (23.1%) and ciprofloxacin (30.8%) showed poor activity. Enterococcus spp. exhibited 40% susceptibility to ampicillin and 50% high-level gentamicin sensitivity. Notably, all Gram-positive isolates were uniformly susceptible to vancomycin and linezolid (100%).

Comparative analysis of MRSA and MSSA isolates (Table 4) showed that children constituted a small proportion of both MRSA (16.7%) and MSSA (12.5%) cases, with the majority occurring in adults. There was no statistically significant association between age group and methicillin resistance (OR 1.40; 95% CI: 0.07–27.9; p = 1.00), and the wide confidence interval reflects the limited sample size.

MRSA isolates demonstrated lower susceptibility across multiple antimicrobial agents compared to MSSA. Although MSSA isolates showed higher susceptibility to clindamycin, erythromycin, doxycycline, ciprofloxacin, gentamicin, and cotrimoxazole, these differences were not statistically significant (p > 0.05). Importantly, both MRSA and MSSA isolates remained uniformly susceptible to vancomycin and linezolid (100%).

A particularly noteworthy finding of the present study is the preserved in vitro activity of colistin against Gram-negative bacilli, including multidrug-resistant pathogens such as Acinetobacter baumannii and Klebsiella pneumoniae, with the latter constituting the majority among isolates tested for colistin susceptibility. The observed minimum inhibitory concentration (MIC) values ranged from 0.125 to 1 µg/mL, with no isolate demonstrating phenotypic resistance.

The predominance of Acinetobacter baumannii in conjunction with its high level of carbapenem resistance underscores its role as a critical driver of multidrug-resistant bloodstream infections in the ICU setting.

Figure 1: Relative burden of major microbial groups in bloodstream infections among MICU patients (%) (GPC: Gram positive cocci, GNB: Gram negative bacilli)

Figure 2: Etiological Spectrum of Bloodstream Infections in a Multidisciplinary Intensive Care Unit

Discussion

The present study delineates the microbial spectrum and antimicrobial resistance patterns of bloodstream infections (BSIs) in a multidisciplinary intensive care unit, highlighting a predominance of Gram-negative pathogens and a high burden of multidrug resistance.

A notable finding of the present study was the overwhelming predominance of Acinetobacter baumannii among bloodstream isolates and the complete preservation of colistin susceptibility despite extensive resistance to multiple antimicrobial classes. This observation provides important contemporary data from North India, where reports describing ICU-specific bloodstream infection epidemiology and colistin susceptibility remain limited.

In our cohort, Gram-negative bacilli (GNB) accounted for the majority of isolates, with Acinetobacter baumannii emerging as the leading pathogen. This finding is consistent with several studies from Indian ICUs, where non-fermenting Gram-negative bacilli, particularly Acinetobacter spp., have increasingly dominated nosocomial infections due to their environmental resilience and remarkable ability to acquire resistance determinants [13,14]. Similar trends have been reported by Veeraraghavan et al. [15] and Shrivastava et al. [16], who documented a rising prevalence of Acinetobacter in bloodstream infections across tertiary care centres. Data from the Indian Council of Medical Research (ICMR) AMR surveillance network also highlight Acinetobacter baumannii as a leading cause of ICU infections with high resistance rates [17].

The high level of resistance to cephalosporins and fluoroquinolones observed in this study reflects the widespread dissemination of extended-spectrum β-lactamase (ESBL)-producing organisms and increasing fluoroquinolone resistance. Although carbapenems and aminoglycosides retained relatively better activity, the susceptibility rates were suboptimal. Of particular concern is the marked multidrug resistance in Acinetobacter baumannii, with carbapenem susceptibility limited to approximately 20%. This is in concordance with global reports of carbapenem-resistant Acinetobacter (CRAB), identified by the World Health Organization as a critical priority pathogen [18]. Comparable findings have been reported in multicentric Indian studies, which demonstrate alarmingly high carbapenem resistance among Acinetobacter isolates [19,20].

Among Enterobacterales, Enterobacter spp. and Klebsiella pneumoniae exhibited moderate susceptibility to carbapenems, suggesting the presence of ESBL production with evolving carbapenem resistance. Studies by Datta et al. ^[21] and Taneja et al. [22] have similarly reported increasing resistance among Enterobacterales, driven by ESBL and carbapenemase production. The emergence of carbapenem-resistant Enterobacterales (CRE) represents a major therapeutic challenge and has been associated with poor clinical outcomes in ICU settings [23]. Pseudomonas aeruginosa in our study demonstrated relatively better susceptibility to aminoglycosides and carbapenems, although persistent resistance to fluoroquinolones was observed, consistent with previously reported resistance patterns [24].

Among Gram-positive organisms, Staphylococcus aureus was the predominant isolate, with an MRSA prevalence of approximately 43%, aligning with reports from Indian ICUs showing MRSA rates ranging from 30% to 50% [25,26]. MRSA isolates demonstrated reduced susceptibility across multiple antimicrobial classes, although statistical significance was not achieved for most agents, likely due to the limited sample size. Importantly, uniform susceptibility to vancomycin and linezolid was observed, consistent with findings from other Indian and global studies indicating preserved activity of these agents [27]. However, increasing reliance on glycopeptides and oxazolidinones raises concerns regarding the potential emergence of resistance, including vancomycin-intermediate S. aureus (VISA) and linezolid resistance [28].

Enterococcus spp., although less frequently isolated, demonstrated reduced susceptibility to ampicillin and variable high-level aminoglycoside resistance, findings comparable to previous Indian studies [29]. The absence of vancomycin resistance in the present study is encouraging; however, the emergence of vancomycin-resistant enterococci (VRE) in tertiary care settings has been increasingly reported and necessitates vigilant surveillance [30].

The high burden of antimicrobial resistance observed in this study can be attributed to factors inherent to ICU settings, including prolonged hospitalization, invasive procedures, device-associated infections, and extensive empirical use of broad-spectrum antibiotics. These factors contribute to significant selective pressure, facilitating the emergence and dissemination of multidrug-resistant organisms (MDROs) [31]. Similar observations have been emphasized in global reports by the Centres for Disease Control and Prevention (CDC) and WHO, highlighting ICU settings as epicentres for AMR emergence [32].

The predominance of carbapenem-resistant Acinetobacter baumannii observed in the present study mirrors a worrying global trend and signals the emergence of a potential post-carbapenem era in critical care medicine. As resistance continues to erode the effectiveness of conventional therapeutic options, clinicians are increasingly compelled to rely on last-resort agents such as colistin. The complete susceptibility to colistin documented in this study offers a narrow but crucial therapeutic window; however, reports of emerging colistin resistance worldwide warrant cautious optimism. Sustained surveillance, rational antimicrobial prescribing, and aggressive infection-control measures are essential to prevent further dissemination of multidrug-resistant organisms and safeguard the efficacy of remaining treatment options.

Limitations

This study is limited by its retrospective, single-centre design, which may affect data completeness and generalizability. The small number of Gram-positive isolates may have reduced the statistical power to detect significant differences. Additionally, molecular characterization of resistance mechanisms and clinical outcome analysis were not performed. Despite these limitations, the study provides valuable institution-specific insights into microbial trends and antimicrobial resistance in ICU bloodstream infections.

Conclusion

This study highlights the emergence of carbapenem-resistant Acinetobacter baumannii as a dominant driver of ICU bloodstream infections and a major contributor to the antimicrobial resistance burden. The retained susceptibility to colistin provides a crucial therapeutic advantage; however, reliance on last-resort agents is unsustainable. Urgent, coordinated efforts in surveillance, stewardship, and infection control are essential to combat the advancing resistance threat.

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