Resistance Patterns in Gram-Negative Bacilli Isolated in a Secondary Care Hospital: A Therapeutic Challenge in Western Mexico

Abstract

Antimicrobial resistance (AMR) continues to represent a significant global public health concern. Gram-negative bacilli (GNB) are the primary causative agents of severe nosocomial infections and possess a notable capacity to develop resistance mechanisms that restrict therapeutic options. The objective of this study was to characterize the antimicrobial susceptibility profiles of GNB isolated at a secondary-level hospital in Guadalajara, Mexico, with the aim of identifying predominant resistance patterns and the most effective therapeutic alternatives. A descriptive, retrospective, cross-sectional study was conducted using clinical isolates of Acinetobacter spp., Pseudomonas spp., Escherichia coli, Klebsiella spp., Morganella morganii, Proteus spp., and Enterobacter spp. collected during 2024. The identification and susceptibility testing were carried out using the VITEK^® 2 automated system, and the results were interpreted in accordance with CLSI guidelines. High resistance rates were observed in Acinetobacter spp. and Pseudomonas spp., particularly to carbapenems (>50% and >40%, respectively). Escherichia coli and Klebsiella spp. demonstrated resistance to third-generation cephalosporins and trimethoprim/sulfamethoxazole, exhibiting high susceptibility to amikacin and carbapenems (>90%). New-generation β-lactam/β-lactamase inhibitor combinations, such as ceftazidime/avibactam and ceftolozane/tazobactam, have demonstrated high efficacy against resistant strains. Overall, GNB isolates in this secondary-level hospital demonstrated elevated resistance levels, particularly to β-lactams and carbapenems, which pose a significant therapeutic challenge. Nevertheless, amikacin, carbapenems, and new-generation β-lactams persist as valuable therapeutic options. In order to contain the spread of multidrug-resistant organisms, it is imperative to strengthen local surveillance, optimize antibiotic stewardship, and reinforce infection control measures.

1. Introduction

Antimicrobial resistance (AMR) remains one of the most pressing global public health challenges of the 21st century. The misuse and overuse of antibiotics, coupled with the horizontal transfer of resistance genes, have accelerated the emergence of multidrug-resistant organisms that compromise the effectiveness of treatments for common infections, resulting in increased morbidity, mortality, and healthcare costs [1,2].

Within this global context, Gram-negative bacilli (GNB) represent a critical clinical threat. Pathogens such as Acinetobacter baumannii, Pseudomonas aeruginosa, and members of the Enterobacterales family (Escherichia coli (E. coli), Klebsiella pneumoniae) are frequently associated with severe nosocomial infections, including ventilator-associated pneumonia, bloodstream infections, and urinary tract infections [3,4,5]. Their success as opportunistic pathogens lies in their remarkable genetic adaptability and the acquisition of multiple resistance mechanisms, including the production of extended-spectrum β-lactamases (ESBLs), AmpC enzymes, and carbapenemases [6].

Particularly alarming is the growing resistance to carbapenems, which are considered last-line agents for the treatment of severe infections caused by multidrug-resistant GNB. The global dissemination of isolates exhibiting extensively drug-resistant (XDR) and pan-drug-resistant (PDR) profiles has severely restricted therapeutic options, often forcing clinicians to resort to toxic alternatives such as colistin or to rely on new β-lactam/β-lactamase inhibitor combinations like ceftazidime/avibactam and ceftolozane/tazobactam [7,8,9].

In Mexico, national surveillance networks such as INVIFAR have reported high prevalence rates of carbapenemase-producing Enterobacterales and non-fermenting GNB, particularly those harboring bla_NDM and bla_OXA48, confirming the critical need for continuous hospital-level monitoring [10]. Similarly, the SMART study reported ESBL production in more than 55% of E. coli and 40% of Klebsiella pneumoniae isolates from intra-abdominal and urinary infections in Mexican hospitals [11].

Recent environmental studies have also identified carbapenemase-producing Klebsiella and Acinetobacter species in hospital and municipal wastewater, suggesting potential environmental dissemination routes that may contribute to the persistence of resistance [12,13]. Therefore, local epidemiological surveillance is essential to guide empirical therapy and strengthen antimicrobial stewardship strategies [14]. In Mexico, available data indicate a rising prevalence of ESBL-producing and carbapenem-resistant Enterobacterales and non-fermenting GNB, but continuous hospital-level surveillance remains scarce [15,16]. In this context, the present study aimed to characterize the antimicrobial susceptibility profiles of Gram-negative bacilli isolated from patients in a secondary-level hospital in Guadalajara, Mexico. By determining the prevalence of ESBL production, carbapenem resistance, and multidrug resistance (MDR/XDR/PDR) patterns, this study seeks to provide updated evidence on local resistance trends and to identify the most effective therapeutic alternatives for clinical management.

2. Materials and Methods

2.1. Study Design and Population

A descriptive, retrospective, cross-sectional study was conducted in which 525 clinical records were analyzed, from which 743 bacterial cultures were obtained from various clinical secretions, including blood cultures, urine cultures, respiratory secretions, and wound exudates. The study was conducted at a second-level hospital located in Guadalajara, Jalisco, during the period between January and December 2024. For the purposes of analysis, only bacterial isolates that were identified as Gram-negative bacilli were considered.

2.2. Bacterial Identification and Antimicrobial Susceptibility Testing

Bacterial species identification and antimicrobial susceptibility testing were performed using the VITEK^® 2 automated system (bioMérieux, Marcy-l’Étoile, France). The corresponding identification and susceptibility cards for Gram-negative bacilli (AST-N233 and AST-N234) were used, always following the manufacturer’s recommendations.

The antibiotics evaluated included a wide range of families, such as penicillins, cephalosporins (first to fourth generation), carbapenems (imipenem, meropenem), aminoglycosides (amikacin, gentamicin), fluoroquinolones (ciprofloxacin, levofloxacin), as well as combinations with beta-lactamase inhibitors (piperacillin/tazobactam, ceftolozane/tazobactam, ceftazidime/avibactam) and other agent such as trimethoprim/sulfamethoxazole.

2.3. Data Analysis and Interpretation of Results

The minimum inhibitory concentration (MIC) results obtained by the VITEK^® 2 system were interpreted as susceptible (S), intermediate (I), or resistant (R) according to the cut-off points established by the Clinical and Laboratory Standards Institute (CLSI) in its M100 guideline, in force during the study period [17].

The classification of the isolates was conducted in accordance with their resistance profile, employing definitions that are consistent with international standards.

The term “multidrug-resistant” (MDR) refers to bacterial isolates that are non-susceptible to at least one agent in at least three categories of antimicrobials.

Extensively drug-resistant (XDR) isolates are susceptible to agents in only one or two categories of antibiotics.

Pan-drug-resistant (PDR) isolates are non-susceptible to all agents in all antimicrobial categories tested [18].

The analysis of the data was conducted through the implementation of descriptive statistics, which entailed the calculation of the frequencies and percentages of resistance for each microorganism-antibiotic combination.

3. Results

3.1. Cross-Species High-Level Resistance Phenotypes

Across Enterobacterales, ESBL positivity was highest in E. coli (59.4%, 95% CI 49.4–68.7; χ² p = 0.013) and substantial in Klebsiella pneumoniae (45.1%, 95% CI 35.2–55.3; p = 0.176), while Klebsiella oxytoca showed a markedly lower rate (10.0%, 95% CI 1.8–40.4; p = 0.009 vs. all other Enterobacterales), see

Table 1. Extended-Spectrum β-Lactamase (ESBL) in Enterobacterales. The table below shows ESBL-positive isolates among selected Enterobacterales species. Proportions and confidence intervals are calculated for each species. The p-value column indicates whether the ESBL positivity rate for a given species differs significantly from the combined rate among all other species.

Species	BLEE Positive	Total Tested	Proportion Positive	CI95 Low	CI95 Upp	p Value
Escherichia coli	57	96	59.4	49.4	68.7	0.013
Klebsiella oxytoca	1	10	10.0	1.8	40.4	0.009
Klebsiella pneumoniae	41	91	45.1	35.2	55.3	0.176

.

Carbapenem non-susceptibility (meropenem) remained low in E. coli (4.6%, 95% CI 2.2–11.6; p < 0.001) and moderate in K. pneumoniae (13.3%, 95% CI 7.0–20.6; p = 0.034), but was significantly higher in non-fermenters—Pseudomonas aeruginosa (41.1%, 95% CI 31.4–52.3; p < 0.001) and Acinetobacter baumannii complex (64.3%, 95% CI 49.2–77.0; p < 0.001), see

Table 2. Carbapenem Resistance (Meropenem). Resistance to meropenem was assessed for common Gram-negative bacilli. The “Resistant” column counts isolates with an interpretation of “Resistant” for meropenem; “Non-resistant” aggregates intermediate and susceptible interpretations. The p-value column tests whether the resistance rate of a species differs from that of the remaining species.

Species	n Tested	R Count	Non R Count	R Prop	CI95 Low	CI95 Upp	p Value
Escherichiacoli	96	5	91	5.2	2.2	11.6	p < 0.001
Klebsiellapneumoniae	90	11	79	12.2	7.0	20.6	0.034
Enterobactercloacae	33	2	31	6.1	1.7	19.6	0.035
Proteushauseri	3	0	3	0.0	0.0	56.1	0.799
Proteusmirabilis	15	0	15	0.0	0.0	20.4	0.089
Morganellamorganii	10	2	8	20.0	5.7	51.0	0.988
Serratiamarcescens	6	0	6	0.0	0.0	39.0	0.421
Citrobacterfreundii	3	1	2	33.3	6.1	79.2	0.569
Citrobacteramalonaticus	1	0	1	0.0	0.0	79.3	0.691
Pseudomonasaeruginosa	82	34	48	41.5	31.4	52.3	p < 0.001
AcinetobacterbaumanniiComplex	42	27	15	64.3	49.2	77.0	p < 0.001

.

When resistance phenotypes were collapsed to MDR/XDR/PDR, resistant categories predominated in E. coli (76.3%, p < 0.001) and A. baumannii complex (66.7%, p = 0.015), while P. aeruginosa showed 32.5% (p = 0.002); smaller series such as Morganella morganii (90.0%, n = 10; p = 0.008) and Serratia marcescens (0%, n = 6; p = 0.036) should be interpreted with caution (

Table 3. Multi-Drug Resistance (MDR/XDR/PDR). Isolates were classified as non-MDR, MDR, XDR or PDR following Magiorakos et al. [18]. To enable statistical comparison, categories MDR/XDR/PDR were aggregated as “Resistant” against “Non-MDR.” The table lists counts of each original category, the overall resistant proportion, and a p-value comparing each species’ resistant proportion against the remainder of the dataset.

Species	Total	Non MDR Count	MDR Count	XDR Count	PDR Count	Resist Prop	p Value
Escherichia coli	97	23	71	3	0	76.3	p < 0.001
Klebsiella pneumoniae	91	48	36	6	1	47.3	0.797
Enterobacter cloacae	33	16	17	0	0	51.5	0.725
Proteus hauseri	3	3	0	0	0	0.0	0.199
Proteus mirabilis	15	8	7	0	0	46.7	0.885
Morganella morganii	10	1	9	0	0	90.0	0.008
Serratia marcescens	6	6	0	0	0	0.0	0.036
Citrobacter freundii	3	2	1	0	0	33.3	0.598
Citrobacter amalonaticus	1	1	0	0	0	0.0	0.709
Pseudomonas aeruginosa	83	56	9	3	15	32.5	0.002
Acinetobacter baumannii Complex	42	14	0	5	23	66.7	0.015

). Collectively, these comparative estimates with 95% confidence intervals and χ² p-values complement the species-specific antibiograms by quantifying between-species differences relevant for empirical therapy and stewardship.

3.2. Acinetobacter spp.

Antimicrobial susceptibility testing of Acinetobacter spp. revealed a widespread pattern of high resistance to multiple clinically used antibiotics. Resistance to cefazolin was nearly universal (97.4%), confirming its minimal efficacy against this pathogen. Piperacillin/tazobactam and third-generation cephalosporins (ceftazidime and ceftriaxone) had high resistance rates (>60%), while cefepime was notable for its lower resistance (20%) but with a high percentage of intermediate results (42.2%), which may limit its clinical use. The carbapenems, Meropenem and Imipenem, had resistance rates exceeding 50%, with a significant proportion of susceptible isolates, indicating a potential role in targeted therapy, contingent upon sensitivity testing. Aminoglycosides (amikacin and gentamicin) had resistance rates exceeding 50%, while trimethoprim/sulfamethoxazole exhibited a high resistance rate of 62.9% (Figure 1).

3.3. Pseudomonas spp.

The susceptibility profile of Pseudomonas spp. exhibited variability in its resistance to the antibiotics evaluated. Cefazolin exhibited complete resistance, thereby confirming its ineffectiveness against this particular pathogen. The new-generation combination of inhibitors, Ceftazidime/Avibactam and Ceftolozane/Tazobactam, demonstrated high efficacy, with more than 74% of isolates being susceptible, reflecting their clinical usefulness against strains resistant to other β-lactams. Among conventional cephalosporins, Ceftazidime and Cefepime showed moderate resistance rates (30.2% and 24.1%, respectively), with a minimal percentage of intermediates, suggesting partial activity. The carbapenems had resistance rates exceeding 40% (Imipenem 46.3%, Meropenem 40.7%), with approximately half of the isolates exhibiting susceptibility, indicating a constrained therapeutic role contingent on sensitivity testing. The aminoglycosides, Amikacin and Gentamicin, exhibited low resistance rates of 22.4% and 14.0%, respectively, and high susceptibility (>76%), thereby underscoring their promise as therapeutic alternatives. Norfloxacin exhibited a comparable trend, with 21.4% resistance and 66.7% susceptible isolates (Figure 2).

3.4. Escherichia coli

Antimicrobial susceptibility testing of E. coli revealed a heterogeneous profile, characterized by high rates of resistance to multiple β-lactams. In contrast, the activity of last-line antibiotics was found to be highly effective. Ampicillin/Sulbactam demonstrated 49% resistance, with 12.5% intermediate results and only 38.5% susceptible isolates. In contrast, Piperacillin/Tazobactam exhibited 80% susceptibility, indicating a more effective response to the antibiotic challenge. Among the new-generation β-lactam inhibitors, both Ceftazidime/Avibactam and Tigecycline exhibited optimal activity, with 100% sensitivity. Conversely, aztreonam and cefalotin exhibited elevated levels of resistance (100% and 70.7%, respectively), thereby constraining their clinical efficacy. Among third-generation cephalosporins, Ceftazidime and Ceftriaxone had resistance rates of 37.1% and 65.5%, respectively, while Cefepime exhibited an intermediate profile with 48.5% resistance and 51.5% susceptibility. Among the carbapenems, Meropenem and Ertapenem exhibited high efficacy, with susceptibility rates of 94.8% and 93.8%, respectively. Fosfomycin and Nitrofurantoin also demonstrated high efficacy, with susceptibility rates exceeding 89%. Among the aminoglycosides, Amikacin had remarkable activity, with a 97.9% susceptibility rate, while Gentamicin exhibited more constrained efficacy, with a 54.3% susceptibility rate. Norfloxacin and Trimethoprim/Sulfamethoxazole exhibited elevated levels of resistance, with 56% and 68.9% of isolates demonstrating resistance, respectively (Figure 3).

3.5. Klebsiella spp.

The resistance profile of Klebsiella spp. exhibited significant variability among the various antibiotic families that were examined. A study of traditional β-lactams revealed that Ampicillin/Sulbactam and Piperacillin/Tazobactam exhibited moderate susceptibility rates of 58% and 60%, respectively. In contrast, aztreonam and cephalothin had high levels of resistance, with percentages of 60% and 47.4%, respectively. Among third- and fourth-generation cephalosporins, Ceftazidime, Ceftriaxone, and Cefepime demonstrated a susceptibility range of 51% to 69%, with a notable proportion of resistant isolates, indicating limited efficacy. Conversely, fosfomycin exhibited high activity (81.1% susceptibility), a performance matched by the carbapenems ertapenem and meropenem, which had sustained efficacy with 88% susceptibility each. Among the aminoglycosides, Amikacin demonstrated the highest level of efficacy, with 98% susceptibility, while Gentamicin exhibited slightly lower effectiveness, with 78.9% susceptibility. Norfloxacin demonstrated consistent activity, with 85.3% of isolates exhibiting susceptibility. Conversely, nitrofurantoin exhibited an unfavorable profile, with more than half of the isolates classified as intermediate and only 35.8% demonstrating susceptibility. Trimethoprim/sulfamethoxazole had high-level resistance (43.2%), with only 56.8% susceptibility. Tigecycline had intermediate activity, with 75% of the isolates exhibiting sensitivity (Figure 4).

3.6. Morganella morganii

The antimicrobial susceptibility profile of Morganella morganii exhibited elevated resistance to multiple conventional β-lactams. Ampicillin/Sulbactam and Cefalotin had resistance rates of 89.9% and 85.7%, respectively, with low susceptibility rates of 11.1% and 14.3%. Conversely, combinations of new-generation β-lactamase inhibitors, such as Ceftazidime/Avibactam and Aztreonam, demonstrated optimal activity with 100% sensitive isolates. Among third-generation cephalosporins, ceftazidime and ceftriaxone demonstrated partial efficacy, exhibiting 90% and 71.4% susceptibility, respectively. The activity of carbapenems had variability, exhibiting 90% susceptibility to Ertapenem and 80% to Meropenem. Among the aminoglycosides, amikacin demonstrated high efficacy (90% susceptibility), while gentamicin exhibited reduced effectiveness (71.4%). In the quinolone group, Norfloxacin had intermediate performance, exhibiting 57.1% sensitivity and 28.6% intermediate isolates. With regard to alternative antimicrobials, fosfomycin, nitrofurantoin, and tigecycline were found to be completely ineffective, exhibiting 100% resistance. Finally, trimethoprim/sulfamethoxazole showed notable resistance, with 71.4% of isolates exhibiting resistance, while only 28.6% were susceptible (Figure 5).

3.7. Proteus spp.

The susceptibility testing of Proteus spp. demonstrated the high efficacy of several antimicrobials that are currently in clinical use. The most recent generation of β-lactamase inhibitors, including Ceftazidime/Avibactam and Ceftolozano/Tazobactam, demonstrated optimal activity with 100% susceptibility, as did carbapenems (Ertapenem and Meropenem) and Amikacin. Among the cephalosporins, Ceftazidime demonstrated an exceptional profile, exhibiting 100% susceptibility. Conversely, Ceftriaxone and Cefepime exhibited high sensitivity rates of 82.4% and 89.5%, respectively. Piperacillin/tazobactam and aztreonam demonstrated moderate activity, with 50% of isolates being susceptible in both cases. In contrast, ampicillin/sulbactam and cephalothin exhibited limited efficacy, with 68.4% and 64.7% susceptibility, respectively. Among the aminoglycosides, Gentamicin and Amikacin demonstrated divergent patterns of activity: while Amikacin exhibited 100% activity, Gentamicin achieved only 70.6% susceptibility, with a notable proportion of intermediate isolates (23.5%). Among the fluoroquinolones, norfloxacin demonstrated a favorable level of efficacy, exhibiting 76.5% susceptibility. Conversely, Nitrofurantoin and Tigecycline exhibited complete ineffectiveness, exhibiting 100% resistance. Trimethoprim/sulfamethoxazole and fosfomycin demonstrated intermediate activity, with 64.7% and 70.6% of isolates exhibiting sensitivity, respectively (Figure 6).

3.8. Enterobacter spp.

The susceptibility profile of Enterobacter spp. exhibited significant variability among the various antibiotic families evaluated. First- and third-generation cephalosporins, including Cefalotin and Ceftriaxone, exhibited high levels of resistance, with percentages of 96.1% and 43.8%, respectively. These findings indicate that these medications have limited efficacy against this species. In contrast, Cefepime, a fourth-generation cephalosporin, exhibited high activity against 91.7% of susceptible isolates, a figure matched by Ceftazidime and Ceftolozane/Tazobactam, with 66.7% and 66.6% sensitivity, respectively. Carbapenems demonstrated remarkable efficacy, particularly Meropenem, which exhibited 94.8% susceptibility, and to a lesser extent, Ertapenem, with 83.3% susceptibility. Among the aminoglycosides, Amikacin demonstrated complete efficacy (100% of susceptible isolates), while Gentamicin exhibited high activity (90.6%). Norfloxacin demonstrated notable efficacy among the fluoroquinolones, exhibiting 96.8% susceptibility, thereby establishing its position as a promising therapeutic alternative. Conversely, Nitrofurantoin exhibited only marginal effectiveness, yielding intermediate results in 50% of cases and demonstrating susceptibility in a mere 21.9% of cases. Trimethoprim/sulfamethoxazole exhibited remarkable activity, with a 90.6% sensitivity rate, while fosfomycin and tigecycline had moderate susceptibility rates of 37.5% and 75%, respectively (Figure 7).

3.9. Cross-Species Analysis of Predictors of Drug Resistance

In the cross-species forest plots (Figure 8A–D), ceftriaxone resistance exceeded one-half of isolates in both E. coli and K. pneumoniae and was highest in the A. baumannii complex; as expected, P. aeruginosa was not tested for ceftriaxone (Figure 8A). For cefepime (Figure 8B), resistance proportions were lower across species than for ceftriaxone, with intermediate values in K. pneumoniae and P. aeruginosa and the lowest estimates in A. baumannii. Fluoroquinolone resistance remained substantial (Figure 8C), approaching ~60% in E. coli and ~65% in A. baumannii, with intermediate levels in K. pneumoniae and P. aeruginosa. Ceftazidime/avibactam retained activity against P. aeruginosa (Figure 8D; resistance ≲25%), whereas Enterobacterales were tested only in small numbers (n ≤ 5 per species) and A. baumannii was not tested; these latter estimates should be interpreted cautiously. Exact counts (R/n) and 95% CIs are annotated in each panel, and species-wise p-values (χ²) are provided in Table 2.

4. Discussion

The findings of this study reveal a concerning scenario of antimicrobial resistance among Gram-negative bacilli (GNB) isolated at a second-level hospital which is consistent with national and international trends [19,20,21]. High resistance rates to β-lactams and carbapenems were observed in Acinetobacter spp. and Pseudomonas spp., confirming their role as predominant pathogens in nosocomial infections that are increasingly difficult to treat. [19,20].

The detection of carbapenem resistance exceeding 60% in Acinetobacter spp. and above 40% in Pseudomonas aeruginosa parallels reports from tertiary care centers in Mexico and Latin America, where OXA-type and NDM-type carbapenemases have been identified as major contributors to resistance [21,22,23,24]. This pattern significantly limits treatment options and frequently necessitates the use of last-resort agents such as colistin, which carry risks of nephrotoxicity and pharmacokinetic challenges [24]. The persistence of XDR and PDR phenotypes in A. baumannii corresponds with the recent identification of DTR (difficult-to-treat resistance) profiles in ESKAPE pathogens isolated in Mexican tertiary hospitals [25]. These findings underscore the rapid evolution of resistance determinants in clinical settings subjected to high antibiotic pressure.

In Pseudomonas spp., resistance heterogeneity reflects both its intrinsic versatility and hospital selection pressures. The preserved susceptibility (>74%) observed for new β-lactam/β-lactamase inhibitor combinations—ceftazidime/avibactam and ceftolozane/tazobactam—highlights their therapeutic potential against multidrug-resistant isolates [26]. However, resistance rates above 40% to carbapenems emphasize the ongoing need for targeted use and strict surveillance of these agents [19,20].

Among the Enterobacterales, E. coli exhibited the highest ESBL positivity (59.4%), followed by K. pneumoniae (45.1%), consistent with reports describing ESBL dissemination as one of the leading causes of β-lactam treatment failure in hospital settings [21,23]. The significant proportion of MDR isolates (76.3%) among E. coli reflects the accumulation of resistance determinants to β-lactams, fluoroquinolones, and trimethoprim/sulfamethoxazole. In contrast, carbapenem resistance remained low (<5%), supporting the continued utility of these agents, alongside amikacin and fosfomycin, for severe and urinary tract infections, respectively.

Klebsiella spp. demonstrated moderate susceptibility to β-lactam/β-lactamase inhibitors and high activity to amikacin (98%) and carbapenems (88–90%), suggesting that these drugs retain efficacy for empiric therapy in severe infections when guided by local antibiograms. Nonetheless, the co-existence of ESBL production and carbapenem resistance in a subset of isolates raises concerns about the possible emergence of carbapenemase-producing Klebsiella strains in the region [27].

For Morganella morganii and Proteus spp., marked resistance to nitrofurantoin and tigecycline was observed, which aligns with their known intrinsic resistance mechanisms. Nevertheless, high susceptibility to carbapenems and amikacin (≥90%) provides reliable therapeutic options. The Enterobacter cloacae complex displayed broad susceptibility to fourth-generation cephalosporins and carbapenems (>90%), but continuous monitoring remains essential given its potential to develop AmpC- and ESBL-mediated resistance [23,24].

Among the Enterobacterales, the predominance of ESBL-positive E. coli and Klebsiella pneumoniae (59.4% and 45.1%, respectively) mirrors nationwide surveillance trends reported by ReLAVRA and INVIFAR, which show consistent increases in ESBL-producing strains across multiple regions of Mexico [19,20,21,28].

The high efficacy observed with amikacin, carbapenems, and new-generation β-lactams suggests that, although useful therapeutic options still exist, their use should be based strictly on susceptibility results and institutional antimicrobial optimization policies [19,22]. The sustained efficacy of amikacin, carbapenems, and new-generation β-lactams against a subset of isolates indicates that, although viable treatment options remain, their use must be guided by susceptibility testing and strict adherence to antimicrobial stewardship protocols. Reinforcing infection control policies, optimizing antibiotic prescribing, and expanding local surveillance programs are critical to curbing the spread of multidrug-resistant organisms [28].

Finally, the emergence of plasmid-mediated colistin resistance (mcr-1 gene) reported in Mexican clinical isolates of E. coli and Klebsiella highlights an additional therapeutic concern, particularly for XDR isolates in which colistin remains one of the few remaining options [29].

In summary, the present study provides updated local evidence on the resistance landscape of Gram-negative bacilli in a secondary hospital in Guadalajara. The identification of high ESBL rates and increasing carbapenem resistance in Acinetobacter and Pseudomonas highlights the urgency of implementing preventive and containment strategies. Incorporating these findings into regional and national surveillance frameworks will be essential to inform clinical decision-making, evaluate intervention effectiveness, and preserve the utility of available antimicrobials in Mexico.

5. Conclusions

The present study demonstrates that Gram-negative bacilli isolated in a tertiary hospital in Guadalajara exhibit elevated levels of resistance to multiple families of antibiotics, particularly among Acinetobacter spp. and Pseudomonas spp., where resistance to carbapenems poses a substantial therapeutic challenge. Similarly, enterobacteria, specifically E. coli and Klebsiella spp., exhibited a high prevalence of phenotypes that are compatible with the production of extended-spectrum beta-lactamases. This phenomenon limits the utilization of broad-spectrum cephalosporins and fluoroquinolones.

Despite this unfavorable outlook, the results demonstrate that certain agents, including amikacin, carbapenems, and combinations of beta-lactams with new-generation beta-lactamase inhibitors, maintain clinical effectiveness against multidrug-resistant strains, thereby positioning them as reserve therapeutic options.

It is imperative to acknowledge the limitations of this study when interpreting the results. Firstly, the retrospective, descriptive, single-centre design of the study limits the generalizability of the findings, given that resistance patterns may differ between regions and levels of care. Furthermore, the identification of bacteria and the assessment of their susceptibility to antimicrobials were conducted exclusively using the VITEK^® 2 system, without the implementation of additional confirmatory methodologies. This approach has the potential to result in discrepancies in the categorization of certain antimicrobials. Moreover, molecular characterization of resistance mechanisms was not performed, which prevented the identification of specific genes or the determination of the possible clonal spread of resistant strains.

These findings underscore the urgent need to strengthen institutional microbiological surveillance programs, implement effective policies for the rational use of antimicrobials, and promote comprehensive strategies for controlling hospital infections. It is imperative to recognize that the containment of the spread of multidrug-resistant strains and the preservation of the efficacy of available antibiotics will only be possible through coordinated and sustained action.