Antimicrobial resistance among Gram-negative bacteria has become a major challenge in hospital practice. Multidrug-resistant organisms are responsible for severe healthcare-associated infections, including pneumonia, bloodstream infection, urinary tract infection, intra-abdominal infection, surgical-site infection, and device-associated infection. Multidrug resistance is commonly defined as acquired non-susceptibility to at least one antimicrobial agent in three or more antimicrobial categories. Important multidrug-resistant Gram-negative pathogens include extended-spectrum β-lactamase-producing Enterobacterales, carbapenem-resistant Enterobacterales, multidrug-resistant Pseudomonas aeruginosa, and carbapenem-resistant Acinetobacter baumannii. Patients with these infections frequently have prolonged hospitalisation, intensive-care-unit admission, previous broad-spectrum antibiotic exposure, invasive devices, recent surgery, immunosuppression, and multiple comorbidities. Delayed administration of an active antibiotic may contribute to treatment failure and death. During the 2016 study period, therapeutic options for resistant Gram-negative infections were limited. Depending on antimicrobial susceptibility, frequently used agents included carbapenems, colistin, polymyxin B, aminoglycosides, tigecycline, fosfomycin, fluoroquinolones, cefoperazone–sulbactam, and piperacillin–tazobactam. Several newer β-lactam–β-lactamase inhibitor combinations currently used for resistant infections were not routinely available in Indian hospitals. Combination antibiotic therapy is used for several reasons. It may increase the probability that at least one active antibiotic is administered, provide pharmacodynamic synergy, enhance bacterial killing, and reduce the emergence of resistance during treatment. Combination therapy may be particularly valuable in critically ill patients, those with septic shock, and infections caused by organisms susceptible to very few antibiotics. However, combination therapy also increases antimicrobial exposure and may result in nephrotoxicity, hepatotoxicity, neurotoxicity, drug interactions, secondary infections, and increased treatment costs. Earlier studies of β-lactam–aminoglycoside therapy did not consistently show a survival advantage over active β-lactam monotherapy and reported increased renal toxicity. Evidence regarding the benefit of combination therapy for multidrug-resistant Gram-negative infections remains conflicting. Outcomes may depend on the infecting organism, infection source, disease severity, pharmacokinetic properties of the antibiotics, adequacy of source control, and whether one or more administered agents are microbiologically active. The present study was undertaken to compare the efficacy and safety of combination antibiotic therapy with monotherapy in patients with culture-confirmed multidrug-resistant Gram-negative infections admitted to a tertiary-care teaching hospital. Aim To compare the efficacy of combination antibiotic therapy with monotherapy in patients with multidrug-resistant Gram-negative bacterial infections. Objectives Primary objective To compare clinical success at completion of treatment between patients receiving combination antibiotic therapy and those receiving monotherapy. Secondary objectives 1. To compare microbiological eradication between the two treatment groups. 2. To compare treatment failure and in-hospital mortality. 3. To compare recurrence, duration of antibiotic treatment, and length of hospital stay. 4. To compare the incidence of adverse drug reactions. 5. To describe the distribution of multidrug-resistant Gram-negative organisms and infection sites. 6. To identify factors independently associated with clinical failure and mortality.

Study design A hospital-based comparative observational study was conducted. Study setting The study was conducted in the Department of Pharmacology, Mamata Medical College & General Hospital, Khammam, Telangana, India. Study duration The study was conducted over nine months, from 1 January 2016 to 30 September 2016. Study population The study included adult patients admitted to the hospital with clinically significant, culture-confirmed infections caused by multidrug-resistant Gram-negative bacteria. Sample size A total of 286 eligible patients were included. Sampling method Consecutive eligible patients identified during the study period were included. Study groups Patients were classified into two groups according to the definitive antimicrobial regimen received after availability of culture and susceptibility results: • Monotherapy group: 132 patients who received one systemic antibiotic with documented in-vitro activity against the causative isolate. • Combination-therapy group: 154 patients who received two or more systemic antibiotics simultaneously, with at least one agent demonstrating in-vitro activity against the causative isolate. Inclusion criteria Patients were included when they fulfilled the following criteria: 1. Age of 18 years or older. 2. Hospitalisation during the study period. 3. Clinical evidence of bacterial infection. 4. Isolation of a Gram-negative organism from a clinically relevant specimen. 5. Isolate classified as multidrug resistant. 6. Receipt of definitive monotherapy or combination therapy for at least 48 hours. 7. Availability of sufficient clinical, microbiological, and treatment information. Exclusion criteria Patients were excluded for: 1. Colonisation without evidence of clinical infection. 2. Contaminated clinical specimens. 3. Duplicate isolates from the same infectious episode. 4. Polymicrobial infection requiring independent treatment for a major Gram-positive or fungal pathogen. 5. Treatment for fewer than 48 hours, except when death occurred within that period. 6. Transfer to another hospital before outcome assessment. 7. Incomplete medical records. 8. Pregnancy or lactation. 9. Age younger than 18 years. 10. Repeat admission for the same infectious episode. Operational definitions Multidrug-resistant organism A Gram-negative bacterial isolate was classified as multidrug resistant when it demonstrated acquired non-susceptibility to at least one agent in three or more clinically relevant antimicrobial categories. Clinical success Clinical success was defined as complete resolution or substantial improvement of infection-related signs and symptoms without the need to add or replace antibiotics because of inadequate response. Clinical failure Clinical failure was defined as one or more of the following: • Persistence or worsening of infection • Requirement for an additional or alternative antibiotic because of inadequate response • Progression to septic shock • Recurrence during the same admission • Infection-attributable death Microbiological eradication Microbiological eradication was defined as absence of the original organism in a repeat culture obtained from the same anatomical site following treatment. Recurrence Recurrence was defined as re-isolation of the same bacterial species from the original infection site after initial clinical improvement or microbiological clearance. Nephrotoxicity Nephrotoxicity was defined as an increase in serum creatinine of at least 0.3 mg/dL within 48 hours or an increase to at least 1.5 times the baseline value during antibiotic therapy. Data collection Data were collected using a structured case-record form. The following information was recorded: • Age and sex • Hospital ward or intensive-care-unit admission • Comorbid conditions • Previous hospitalisation • Previous antimicrobial exposure • Recent surgery • Presence of invasive devices • Mechanical ventilation • Infection source • Presence of sepsis or septic shock • Microbiological isolate • Antimicrobial-susceptibility pattern • Empirical and definitive antibiotic regimens • Time to administration of an active antibiotic • Duration of therapy • Source-control procedure • Clinical outcome • Repeat culture result • Adverse drug reactions • Length of hospital stay • In-hospital mortality Microbiological methods Clinical specimens were processed according to standard microbiological procedures. Organisms were identified using colony morphology, Gram staining, and conventional biochemical tests. Antimicrobial-susceptibility testing was performed using the Kirby–Bauer disc-diffusion method on Mueller–Hinton agar. Results were interpreted according to the Clinical and Laboratory Standards Institute criteria applicable during 2016. The following antibiotics were tested according to the organism and specimen type: • Amikacin • Gentamicin • Ciprofloxacin • Levofloxacin • Ceftriaxone • Ceftazidime • Cefepime • Piperacillin–tazobactam • Cefoperazone–sulbactam • Imipenem • Meropenem • Tigecycline • Colistin Antibiotic regimens Monotherapy regimens included: • Carbapenem monotherapy • Colistin monotherapy • Aminoglycoside monotherapy • Tigecycline monotherapy • Cefoperazone–sulbactam monotherapy • Piperacillin–tazobactam monotherapy • Other susceptibility-guided monotherapy Combination regimens included • Colistin plus meropenem • Colistin plus tigecycline • Colistin plus an aminoglycoside • Meropenem plus an aminoglycoside • Cefoperazone–sulbactam plus an aminoglycoside • Other two- or three-drug regimens Antibiotic selection and dose adjustment were determined by the treating physicians according to susceptibility results, infection site, disease severity, renal function, and clinical response. Outcome assessment Clinical response was assessed: 1. At initiation of definitive treatment 2. After 48–72 hours 3. At completion of treatment 4. At discharge or death Ethical considerations Patient confidentiality was maintained throughout the study. Because the study involved observational analysis of routine clinical treatment, no intervention was assigned by the investigators. Statistical analysis • Data were entered into Microsoft Excel and analysed using IBM SPSS Statistics version 20.0. • Continuous variables were expressed as mean ± standard deviation or median with interquartile range. Categorical variables were presented as frequencies and percentages. • The chi-square test or Fisher’s exact test was used to compare categorical variables. The independent-samples t test was used for normally distributed continuous variables, and the Mann–Whitney U test was used for non-normally distributed variables. • Risk ratios, odds ratios, and 95% confidence intervals were calculated where appropriate. Variables associated with clinical outcome at p <0.20 in univariate analysis, together with clinically important variables, were included in multivariable logistic-regression analysis. • A two-sided p value below 0.05 was considered statistically significant.

Patient characteristics A total of 286 patients with multidrug-resistant Gram-negative infections were included. The mean age was 53.8 ± 16.4 years. There were 174 male patients (60.8%) and 112 female patients (39.2%). Of the 286 patients • 132 patients (46.2%) received monotherapy. • 154 patients (53.8%) received combination therapy. The combination-therapy group contained higher proportions of patients admitted to the intensive-care unit, receiving mechanical ventilation, and presenting with septic shock. Table 1. Baseline characteristics Characteristic Monotherapy, n = 132 Combination therapy, n = 154 p value Age, mean ± SD, years 52.1 ± 16.2 55.3 ± 16.5 0.102 Male sex 78 (59.1%) 96 (62.3%) 0.576 Intensive-care admission 48 (36.4%) 79 (51.3%) 0.011 Diabetes mellitus 49 (37.1%) 63 (40.9%) 0.513 Chronic kidney disease 18 (13.6%) 25 (16.2%) 0.541 Cardiovascular disease 31 (23.5%) 42 (27.3%) 0.464 Malignancy 10 (7.6%) 15 (9.7%) 0.518 Previous antibiotic exposure 71 (53.8%) 101 (65.6%) 0.042 Mechanical ventilation 31 (23.5%) 55 (35.7%) 0.025 Septic shock 19 (14.4%) 39 (25.3%) 0.022 Bacteraemia 27 (20.5%) 42 (27.3%) 0.181 Adequate source control 90 (68.2%) 108 (70.1%) 0.725 Active therapy within 24 hours 80 (60.6%) 109 (70.8%) 0.070 Infection sites Respiratory-tract infections were the most common, followed by urinary-tract infections and bloodstream infections. Table 2. Distribution of infection sites Infection site Monotherapy Combination therapy Total Respiratory tract 34 55 89 (31.1%) Urinary tract 41 37 78 (27.3%) Bloodstream 23 37 60 (21.0%) Intra-abdominal 12 10 22 (7.7%) Surgical site 11 8 19 (6.6%) Skin and soft tissue 7 5 12 (4.2%) Other 4 2 6 (2.1%) Total 132 154 286 (100%) Microbiological profile Klebsiella pneumoniae was the most frequently isolated organism, accounting for 94 infections (32.9%). This was followed by Escherichia coli in 72 patients (25.2%), Acinetobacter baumannii in 62 (21.7%), and Pseudomonas aeruginosa in 44 (15.4%). Table 3. Distribution of multidrug-resistant organisms Organism Monotherapy Combination therapy Total Klebsiella pneumoniae 39 55 94 (32.9%) Escherichia coli 43 29 72 (25.2%) Acinetobacter baumannii 22 40 62 (21.7%) Pseudomonas aeruginosa 20 24 44 (15.4%) Enterobacter species 5 4 9 (3.1%) Other Gram-negative organisms 3 5 8 (2.8%) Total 132 154 286 Resistance phenotypes Extended-spectrum β-lactamase production was identified in 119 isolates (41.6%). Carbapenem resistance was present in 102 isolates (35.7%), and 65 isolates (22.7%) demonstrated other multidrug-resistant phenotypes without documented carbapenem resistance. Carbapenem resistance was more frequent in the combination-therapy group than in the monotherapy group: 67 of 154 patients (43.5%) versus 35 of 132 patients (26.5%; p = 0.003). Definitive antibiotic treatment Carbapenems were the most frequently used monotherapy agents. Colistin plus meropenem was the most common combination regimen. Table 4. Definitive antibiotic regimens Antibiotic regimen Number Percentage Monotherapy Carbapenem 49 17.1% Cefoperazone–sulbactam 28 9.8% Piperacillin–tazobactam 17 5.9% Colistin 14 4.9% Aminoglycoside 13 4.5% Tigecycline 7 2.4% Other monotherapy 4 1.4% Combination therapy Colistin plus meropenem 61 21.3% Colistin plus tigecycline 28 9.8% Meropenem plus aminoglycoside 25 8.7% Colistin plus aminoglycoside 17 5.9% Cefoperazone–sulbactam plus aminoglycoside 13 4.5% Other combinations 10 3.5% Total 286 100% Primary outcome Clinical success was achieved in 77 of 132 patients receiving monotherapy (58.3%) and 111 of 154 patients receiving combination therapy (72.1%). The absolute difference in clinical success was 13.8 percentage points. Combination therapy was associated with a higher probability of clinical success than monotherapy: • Risk ratio: 1.24 • 95% confidence interval: 1.03–1.48 • p = 0.021 Secondary outcomes Microbiological eradication occurred in 61 patients receiving monotherapy (46.2%) and 93 receiving combination therapy (60.4%; p = 0.023). In-hospital mortality was numerically higher with monotherapy, but the difference was not statistically significant: • Monotherapy: 25 of 132 patients (18.9%) • Combination therapy: 20 of 154 patients (13.0%) • p = 0.224 Table 5. Clinical and microbiological outcomes Outcome Monotherapy, n = 132 Combination therapy, n = 154 p value Clinical success 77 (58.3%) 111 (72.1%) 0.021 Clinical failure 55 (41.7%) 43 (27.9%) 0.021 Microbiological eradication 61 (46.2%) 93 (60.4%) 0.023 Persistent positive culture 26 (19.7%) 23 (14.9%) 0.286 Follow-up culture unavailable 45 (34.1%) 38 (24.7%) 0.080 In-hospital mortality 25 (18.9%) 20 (13.0%) 0.224 Recurrence 14 (10.6%) 12 (7.8%) 0.536 Median treatment duration, days 10 (8–14) 14 (10–17) <0.001 Median hospital stay, days 15 (11–21) 18 (13–25) 0.004 Any adverse drug reaction 14 (10.6%) 30 (19.5%) 0.038 Nephrotoxicity 8 (6.1%) 21 (13.6%) 0.055 Hepatotoxicity 3 (2.3%) 5 (3.2%) 0.724 Neurotoxicity 1 (0.8%) 2 (1.3%) 1.000 Antibiotic discontinued because of toxicity 2 (1.5%) 5 (3.2%) 0.457 The longer treatment duration and hospital stay in the combination group were likely influenced by greater baseline disease severity. Organism-specific clinical success Combination therapy was associated with numerically higher clinical success for infections caused by K. pneumoniae, A. baumannii, and P. aeruginosa. The largest difference was observed among patients with A. baumannii infection. Table 6. Clinical success according to organism Organism Monotherapy Combination therapy p value K. pneumoniae 23/39 (59.0%) 40/55 (72.7%) 0.164 E. coli 29/43 (67.4%) 22/29 (75.9%) 0.439 A. baumannii 9/22 (40.9%) 28/40 (70.0%) 0.025 P. aeruginosa 11/20 (55.0%) 17/24 (70.8%) 0.276 Other organisms 5/8 (62.5%) 4/6 (66.7%) 1.000 Outcomes according to infection site Clinical success was higher with combination therapy among patients with respiratory-tract and bloodstream infections. No significant difference was observed among patients with urinary-tract infections. Table 7. Clinical success according to infection site Infection site Monotherapy Combination therapy p value Respiratory tract 17/34 (50.0%) 39/55 (70.9%) 0.047 Urinary tract 29/41 (70.7%) 29/37 (78.4%) 0.441 Bloodstream 11/23 (47.8%) 26/37 (70.3%) 0.082 Other sites 20/34 (58.8%) 17/25 (68.0%) 0.472 Adverse drug reactions At least one adverse drug reaction occurred in 44 patients (15.4%). Adverse reactions were more frequent in the combination-therapy group: • Monotherapy: 14 of 132 patients (10.6%) • Combination therapy: 30 of 154 patients (19.5%) • p = 0.038 Nephrotoxicity was the most common adverse reaction and occurred predominantly in patients receiving colistin- or aminoglycoside-containing regimens. Factors associated with clinical failure On univariate analysis, clinical failure was associated with: • Septic shock • Mechanical ventilation • Bacteraemia • Delayed active therapy • Inadequate source control • Chronic kidney disease • Monotherapy Table 8. Multivariable logistic-regression analysis of clinical success Variable Adjusted odds ratio 95% confidence interval p value Combination therapy 1.79 1.04–3.09 0.036 Active treatment within 24 hours 2.13 1.22–3.72 0.008 Adequate source control 2.41 1.36–4.26 0.003 Septic shock 0.34 0.17–0.66 0.002 Mechanical ventilation 0.48 0.27–0.87 0.015 Bacteraemia 0.61 0.34–1.10 0.101 Chronic kidney disease 0.58 0.29–1.18 0.132 Carbapenem-resistant organism 0.69 0.39–1.22 0.204 After adjustment for potential confounders, combination therapy remained independently associated with clinical success. Early administration of active therapy and adequate source control were also independent predictors of clinical success. Septic shock and mechanical ventilation were associated with a lower probability of clinical success. Factors associated with mortality Table 9. Multivariable logistic-regression analysis of in-hospital mortality Variable Adjusted odds ratio 95% confidence interval p value Combination therapy 0.72 0.36–1.44 0.351 Age ≥65 years 1.86 0.96–3.60 0.066 Septic shock 4.12 2.01–8.46 <0.001 Mechanical ventilation 2.54 1.24–5.21 0.011 Bacteraemia 1.73 0.87–3.45 0.119 Active treatment within 24 hours 0.49 0.25–0.96 0.038 Adequate source control 0.43 0.21–0.88 0.021 Chronic kidney disease 1.67 0.77–3.62 0.194 Combination therapy was not independently associated with a statistically significant reduction in mortality. Septic shock and mechanical ventilation were the strongest predictors of death.

The present study compared the efficacy and safety of combination antibiotic therapy with monotherapy in 286 patients with multidrug-resistant Gram-negative infections. Clinical success was significantly higher in the combination-therapy group than in the monotherapy group. Combination therapy was associated with an absolute increase of 13.8 percentage points in clinical success. Microbiological eradication was also significantly more frequent with combination therapy. Although mortality was lower in the combination-therapy group, the difference was not statistically significant. This suggests that improved clinical response and bacterial clearance may not necessarily translate into a measurable survival advantage, particularly in a heterogeneous population in which mortality is influenced by age, comorbidities, septic shock, organ failure, infection source, and source control. The combination group had higher baseline disease severity. More patients receiving combination therapy required intensive-care admission, mechanical ventilation, and treatment for septic shock. They were also more likely to have previous antibiotic exposure and carbapenem-resistant infections. Despite these adverse baseline characteristics, combination therapy remained independently associated with clinical success after multivariable adjustment. The possible benefit of combination therapy may be explained by several mechanisms. The use of two active or partially active agents may improve bacterial killing, provide broader antimicrobial coverage, and reduce the likelihood of initial inadequate treatment. Certain antibiotic combinations may demonstrate pharmacodynamic synergy, particularly against organisms with high minimum inhibitory concentrations or heteroresistant subpopulations. The greatest organism-specific difference was observed among patients with multidrug-resistant A. baumannii. Clinical success was achieved in 70.0% of patients receiving combination therapy compared with 40.9% receiving monotherapy. However, this was a subgroup analysis involving a limited number of patients and should be interpreted cautiously. Combination therapy was also associated with improved outcomes in respiratory-tract infection. Pneumonia caused by resistant Gram-negative organisms is difficult to treat because of high bacterial burden, variable pulmonary penetration, critical illness, and frequent mechanical ventilation. The use of two agents may have increased the probability of achieving an effective concentration at the infection site. No significant difference was observed in urinary-tract infections. High urinary concentrations achieved by several antibiotics may make active monotherapy sufficient for many urinary infections, particularly when bacteraemia or obstruction is absent. Early administration of active antibiotic treatment was independently associated with clinical success and reduced mortality. This finding emphasises the importance of rapid microbiological diagnosis, knowledge of local resistance patterns, and prompt modification of empirical therapy. Adequate source control was another independent predictor of clinical success and survival. Antibiotic escalation cannot compensate for an undrained abscess, obstructed urinary tract, infected catheter, necrotic tissue, or uncontrolled intra-abdominal source. The potential benefits of combination therapy must be balanced against toxicity. Any adverse drug reaction was significantly more common in the combination group. Nephrotoxicity occurred in 13.6% of patients receiving combination therapy compared with 6.1% receiving monotherapy. The difference narrowly missed conventional statistical significance but was clinically important. Colistin and aminoglycosides were frequently included in combination regimens during the study period. Both classes can cause renal injury, particularly in older patients, critically ill patients, and those receiving other nephrotoxic drugs. Renal-function monitoring and dose adjustment are therefore essential. The combination group also had longer treatment duration and hospital stay. This should not automatically be interpreted as an adverse effect of combination therapy because these patients had more severe disease at baseline. Residual confounding may remain despite statistical adjustment. The findings do not support routine combination treatment for every multidrug-resistant Gram-negative infection. Patients with lower-severity urinary infection and a reliably active single agent may not derive additional benefit. Combination therapy may be most appropriate for septic shock, severe pneumonia, bloodstream infection, carbapenem-resistant pathogens, delayed susceptibility results, or situations in which the activity of a single agent is uncertain. Once susceptibility results are available and the patient is clinically stable, de-escalation to the narrowest effective regimen should be considered. This approach may preserve clinical efficacy while reducing nephrotoxicity, drug interactions, treatment costs, and selective pressure for further resistance. Strengths The study had several strengths: 1. It included a relatively large sample of 286 patients. 2. It reflected real-world prescribing in a tertiary-care teaching hospital. 3. Both clinical and microbiological outcomes were evaluated. 4. Multiple infection sites and Gram-negative organisms were included. 5. Adverse drug reactions were compared between treatment groups. 6. Multivariable analysis was used to adjust for important differences in disease severity. 7. The study evaluated treatment practices during a period when therapeutic options for multidrug-resistant infections were limited. Limitations • The study had several limitations. • First, the observational design was susceptible to confounding by indication. Combination therapy was more frequently administered to critically ill patients and those with carbapenem-resistant organisms. • Second, the study was conducted at a single centre, which may limit generalisability. • Third, the antibiotic regimens were heterogeneous. Different combinations may not have equivalent efficacy or toxicity. • Fourth, molecular characterisation of resistance mechanisms was not routinely available. • Fifth, follow-up cultures were not available for every patient. • Sixth, minimum inhibitory concentrations and therapeutic drug monitoring were not consistently available. • Seventh, treatment selection, dosing, source-control practices, and timing of follow-up cultures were determined by the treating teams and were not standardised by a study protocol. • Eighth, the study duration was limited to nine months and may not reflect seasonal or long-term changes in antimicrobial resistance. • Ninth, several newer antimicrobial agents now used for resistant Gram-negative infections were not routinely available during the 2016 study period. • Finally, long-term mortality and post-discharge recurrence were not systematically assessed

In this comparative observational study of 286 patients with multidrug-resistant Gram-negative infections, combination antibiotic therapy was associated with significantly higher clinical-success and microbiological-eradication rates than monotherapy. Combination therapy did not produce a statistically significant reduction in in-hospital mortality. It was also associated with a higher incidence of adverse drug reactions, particularly nephrotoxicity. The results suggest that combination therapy may be beneficial in selected patients with severe infection, septic shock, respiratory or bloodstream infection, carbapenem-resistant pathogens, or limited therapeutic options. Active monotherapy may remain appropriate for less severe infections when a reliably active antibiotic with adequate tissue penetration is available. Early administration of active treatment, appropriate source control, antimicrobial-susceptibility-guided therapy, renal-dose adjustment, and careful monitoring for toxicity remain essential components of successful management. Recommendations 1. Combination therapy should be considered in critically ill patients with a high probability of multidrug-resistant Gram-negative infection. 2. Empirical combination treatment should be reassessed after culture and susceptibility results become available. 3. De-escalation to active monotherapy should be considered in clinically stable patients. 4. Renal function should be monitored closely when colistin or aminoglycosides are used. 5. Adequate source control should be prioritised. 6. Hospital-specific antibiograms should guide empirical treatment. 7. Antimicrobial-stewardship review should be incorporated into management. 8. Prospective multicentre studies should evaluate organism- and regimen-specific outcomes.

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