The last 20 years have been characterized by the emergence and spread of antibiotic-resistant bacteria, which in turn have become a determining factor for the failure of antibiotic therapies, mainly in patients admitted to intensive care units (ICU). ^1,2 This public health problem has serious implications on morbidity and mortality rates, and on public health systems stability, provided that both direct and indirect economic costs increase heavily due long-term hospitalizations and greater drug consumption. ³

To date, different strategies have been developed to minimize the impact this public health problem can have on health systems, including appropriate antibiotics prescription, cleaning and disinfection programs, antimicrobial stewardship programs, and epidemiological surveillance of antimicrobial resistance. ^2,4

It has been estimated that in patients admitted to a hospital the risk of contracting a nosocomial infection is ~10%. In addition, said risk almost quadruples when the individual is admitted to the ICU, since antimicrobial therapy is a common practice in ICU patients. ^5,6 Moreover, multidrug-resistant bacteria infection susceptibility may be higher in these critically ill patients ^7-9, which limits treatment options and makes mandatory to determine antimicrobial resistance profiles among clinical isolates retrieved from said patients.

Given how antimicrobial resistance works, any modification to be made in antimicrobial management schemes should be based on both antimicrobial resistance surveillance data and bacterial etiology. ¹⁰

Changes made to the antimicrobial management schemes are not generally applicable to other hospitals, for they are based on the intrinsic characteristics of each hospital and depend on several aspects such as the type of hospital (facilities), antibiotics availability, and specific local data obtained from monitoring antimicrobial resistance. ¹¹

Staphylococcus spp. are frequently cultured from hospitalized patients and are characterized by being resistant to multiple antimicrobials. Methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant Staphylococcus epidermidis (MRSE) can be used as markers to evaluate antimicrobial resistance time trends in critically ill patients. ^12-19

Espinosa et al.²⁰, in a systematic review on antimicrobial resistance in gram-positive cocci in Colombian hospitals, reported a higher isolation frequency of methicillin-resistant staphylococci in ICU isolates in comparison to non-ICU isolates. ²⁰ This finding is consistent with those of Villalobos et al.¹⁶ and Villalobos et al.²¹, who reported MRSA antibiotic resistance phenotypes and profiles in tertiary hospitals in Colombia. In this context, the importance of an active in vitro antimicrobial resistance surveillance program should be noted.

Effective surveillance of antimicrobial resistant organisms allows generating reports that provide necessary information for analyzing and monitoring resistance patterns; likewise, these surveillance data can be used for making decisions regarding the treatment. ²²

The objective of the present study was to evaluate the temporal trends of MRSA and MRSE antibiotic resistance phenotypes from isolates obtained between 2010 and 2015 from ICU patients in a tertiary hospital located in Cartagena, Colombia.

A total of 313 staphylococcal strains were isolated and identified, of which 63.6% (199/313; 95%CI 58.1- 68.7) were methicillin-resistant staphylococcal isolates (Table 1: MRSA+MRSE + other MR-CoNS). Said isolates were obtained from patients admitted to the adult ICU (AICU) (57.1%; 93/163; 95%CI: 49.4- 64.4), the neonatal ICU (NICU) (78.8%; 67/85; 95%CI: 69.0-86.2), and the pediatric ICU (PICU) (60.0%; 39/65; 95%CI: 47.9-71.0) (Table 1).

Table 1

AICU: adult intensive care unit; PICU: pediatric intensive care unit; NICU: neonatal intensive care unit; MSSA: methicillin-susceptible S. aureus; MRSA: methicillin-resistant S. aureus; MSSE: methicillin-susceptible S. epidermidis; MRSE: methicillin-resistant S. epidermidis; MS-CoNS: methicillin-susceptible coagulase-negative staphylococci; MR-CoNS: methicillin-resistant coagulase-negative staphylococci.

According to the six years susceptibility test results , methicillin resistance rates of S. aureus and S. epidermidis for the whole period were 45.7% (43/94; 95%CI: 36.0-55.8) and 75.0% (87/116; 95%CI: 66.4-82.0), respectively.

Regarding species identification, by using the automated identification system it was found that 53.0% (116/219; 95%CI: 43.4-59.5) of the coagulase-negative Staphylococcus (CoNS) that were tested belonged to the S. epidermidis species, while 15.5% (34/219; 95%CI: 11.3-20.9), to S. hominis subsp hominis, and 11.4% (25/219; 95%CI: 7.9-16.3) to S. haemolyticus. The remaining 20.1% of isolates belonged to other coagulase-negative staphylococci (CoNS) species, including S. auricularis, S. hominis subsp novobiosepticum, S. warneri, S. lugdunensis, S. simulans, S. xylosus, S. intermedius, S. sciuri, S. capitis subsp capitis, and S. capitis subsp urealyticus.

Resistance to methicillin was detected in 71.2% (156/219; 95%CI: 64.9-76.8) of the CoNS isolates. In addition, 13.7% (43/313; 95%CI: 10.4-18.0) and 27.8% (87/313; 95%CI: 23.1-33.0) isolates were MRSA and MRSE, respectively (Table 1).

Sample-based MRSA, MSSA, MRSE, and MSSE isolates distribution is shown in Table 2. Most of these strains (52.9 to 96.6%) were isolated from blood cultures, while the remaining ones were obtained from orotracheal and ocular secretions (<30%), and other body fluids (< 10%).

Table 2

MSSA: methicillin-susceptible S. aureus; MRSA: methicillin-resistant S. aureus; MSSE: methicillin-susceptible S. epidermidis; MRSE: methicillin-resistant S. epidermidis.

Antimicrobial resistance profiles are shown in Table 3. All staphylococcal isolates were identified as sensitive to daptomycin (DAP). Regarding MRSA and MRSE isolates, the highest resistance rates were found for erythromycin (ERI) (57.6% and 81.2%, respectively), clindamycin (CLI) (54.6% and 71.0%, respectively), ciprofloxacin (CIP) (48.4% and 36.4%, respectively) and trimethoprim-sulfamethoxazole (SXT) (36.4% and 51.4%, respectively).

Table 3

MSSA: methicillin-susceptible S. aureus; MRSA: methicillin-resistant S. aureus; MSSE: methicillin-susceptible S. epidermidis; MRSE: methicillin-resistant S. epidermidis; CLI: clindamycin; CIP: ciprofloxacin; DAP: daptomycin; ERI: erythromycin; GEN: gentamicin; LZD: linezolid; MXF: moxifloxacin; NIT: nitrofurantoin; RIF: rifampin; SXT: trimethoprim-sulfamethoxazole; VAN: vancomycin.

In addition, resistance percentages to ERI, CLI, gentamicin (GEN), CIP, and SXT were far higher in in MRSA than in MSSA isolates. A similar behavior was only observed between MRSE and MSSE isolates (Table 4).

Table 4

MSSA: methicillin-susceptible S. aureus; MRSA: methicillin-resistant S. aureus; MSSE: methicillin-susceptible S. epider-midis; MRSE: methicillin-resistant S. epidermidis; CLI-R: clindamycin-resistant; CIP-R: ciprofloxacin-resistant; ERI-R: erythromycin-resistant; GEN-R: gentamicin-resistant; SXT-R: trimethoprim-sulfamethoxazole-resistant.

Concerning MSSA strains, all resistance percentage values were <20%; on the contrary, resistance percentages to ERI and SXT in MSSE isolates were comparable with the high percentage values observed in MRSA isolates (p=0.41; X²=0.68 and p=0.296; X²=1.1, respectively).

Vancomycin MIC values for 2 MRSA isolates (4.7%; 95%CI: 1.315.5) were 8 μg/mL, vancomycin-intermediate S. aureus (VISA), and 16 μg/mL, vancomycin-resistant S. aureus (VRSA), while in 2 MRSE isolates (2.3%; 95%CI: 0.6-8.0) vancomycin MIC values were >16 μg/mL.

Resistance phenotypes trends in MRSA and MRSE isolates were higher in the last year of the study (January-December 2015), with the exception of resistance to SXT, since SXT-resistant isolates were not identified during this period. On the other hand, for the whole six years of the study, the lowest resistance percentages in MRSA and MRSE isolates were found for SXT (27.9% and 42.5%, respectively) (Table 5).

Table 5

MSSA: methicillin-susceptible S. aureus, MRSA: methicillin-resistant S. aureus; MSSE: methicillin-susceptible S. epidermidis; MRSE: methicillin-resistant S. epidermidis; CLI-R: clindamycin-resistant; CIP-R: ciprofloxacin-resistant; ERI-R: erythromycin-resistant; GEN-R: gentamicin-resistant; SXT-R: trime-thoprim-sulfamethoxazole-resistant. Note: Multiresistant strains were included individually for each antibiotic evaluated.

Furthermore, in the case of methicillin-resistant isolates, a high frequency of multi-drug resistant strains was observed (MRSA: 26/43; 60.5%; 95%CI: 45.6-73.6; MRSE: 68/87; 78.2%; 95%CI: 68.4-85.6). In this regard, 28 multiresistant phenotypes (MR-ANT) were identified in 26 MRSA (12 MR-ANT) and 68 MRSE (16 MR-ANT) strains. The MR-Ant frequency order was as follows: CLI-CIP-ERI-GEN-SXT(MRSA: 19.2%; 5/26; 95%CI: 8.5-37.9; MRSE: 23.5%; 16/68; 95%CI: 15.0- 34.9), CLI-CIP-ERI (MRSA: 23.1%; 6/26; 95%CI: 11.0-42.1), CLI-ERI-GEN (MRSE: 14.7%; 10/68; 95%CI: 8.2-25.0), and CLI-ERI-GEN-SXT (MRSE: 11.8%; 8/68; 95%CI: 6.1-21.5).

Multiple antibiotic resistances were also observed: 9 isolates (MRSA: 2/43; 4.7%; 95%CI: 1.3-15.5; MRSE: 7/87; 8.1%; 95%CI: 4.0-15.7) were resistant to a combination of seven antibiotics; 22 (MRSA: 6/43; 14.0%; 95%CI: 6.6-27.3; MRSE: 16/87; 18.4%; 95%CI: 11.7- 27.8), to a six antibiotics combination, and 13 (MRSA: 1/43; 2.3%; 95%CI: 0.4-12.1; MRSE: 12/87; 13.8%; 95%CI: 8.122.6), to a five antibiotics combination.

The highest MAR index values observed in the methicillin-resistant Staphylococcus species isolates ranged from 0.8 to 0.9, while in methicillin-susceptible strains these ranged from 0.2 and 0.4. In addition, in most methicillin-resistant isolates MAR index was higher than 0.2 (MRSE: 79.3%; 69/87; 95CI 69.7 to 86.5%; MRSA: 60.5%; 26/43; 95%CI: 45.6-73.6), in contrast with methicillin-susceptible isolates, where most of them had a MAR index value lower than 0.2 (MSSE: 24.1%; 7/29; 95%CI: 12.2-42.1 ;MSSA: 3.9%;2/51; 95%CI: 1.1-13.2).

On the other hand, the analysis of the methicillin-resistance trend of S. aureus isolates allowed finding a significant increase (p=0.031; X² lineal trend=4.673; 1 df; slope= 1.72) during the six years study period, somehow this epidemiological characteristic was not observed in other bacterial strains for the same period (Chi-square for lineal trend, MSSA: p=0.732; MRSE: p=0.082; MSSE: p=0.323) (Figure 1).

Figure 1

Staphylococcus species have emerged as significant pathogens in both hospital-acquired and community-onset infections. ^28-31 Hospital isolates of bacterial strains tend to have higher rates of resistance to antimicrobial agents, especially those collected from ICU patients. ²⁰

In this study, a higher frequency of methicillin-resistant staphylococci clinical isolates was reported in comparison with methicillin-susceptible isolates, which is consistent with the findings of national ^16,32-38 and international studies. ^12,39-44 In part, this could be explained by the relationship between antibiotic selective pressure and the emergence of antimicrobial-resistant organisms in hospitals. ^45,46

Most methicillin-resistant strains, mainly MRSE, were isolated from patients who were admitted to the NICU. This finding is consistent with several works reporting that S. epidermidis is one of the most common bacterial pathogens in this population. ^{32,39,41,43,47-51} Major risk factors for presenting these staphylococcal infections include prematurity, invasive procedures, prolonged hospitalization, and immune immaturity. ^52,53

On the other hand, the percentage values of MRSA and MRSE strains isolated from blood cultures are similar to those reported by Cortes et al.³⁴ in bacteremic patients admitted to the ICUs of 33 Colombian hospitals.

All these similarities suggest three possible scenarios at a national level: (i) failure of empirical antibacterial therapy ^34,54, (ii) increased hospital stay lengths for clinically ill patients ⁵⁵, and increased risk of mortality in ICU patients. ^34,56

Also, it was found that MRSE isolates showed a higher resistance frequency to antistaphylococcal antibiotics such as ERI, CLI and SXT that MRSA strains did. Somehow, in the case of ciprofloxacin, this pattern was not observed. Similar results were described by Alvarez et al.⁵⁷ In an attempt to explain these findings, the authors of the present study suggest two key factors: selective pressure caused by antibiotic use and cross-transmission of resistant bacteria.

Significant differences between antimicrobial resistance rates of methicillin-resistant and methicillin-susceptible Staphylococcus isolates have also been reported by other authors, although they have done so when comparing coagulase-negative Staphylococcus strains isolated from septicemic patients. ³¹

About a third of the methicillin-resistant isolates were also resistant to other antibiotics including macrolides, lincosamides, quinolones, aminoglycosides, rifamycins, oxazolidinones, nitrofurans and antifolates. In addition, if a MAR index value >0.2 can be considered a multiple antimicrobial resistance characteristic ⁵⁸, the presence of these multidrug-resistant strains could support the emergence of the three hypothetical scenarios described above.

Out of 43 MRSA isolates, 2 vancomycin-intermediate and vancomycin-resistant strains were identified in adult bacteremia cases. To date, there are no reports of VISA or VRSA isolates in Colombian hospitals. ^{21,33,35-37,49,57} However, one study conducted in Caldas, Colombia, in 1996 ³⁸, reported the existence of VRSA strains isolated from patients admitted to the ICU, but these isolates were identified using the disc diffusion method. Despite the fact that VRSA isolates presence has not been confirmed by a reference laboratory, this finding could be interpreted as an early warning of the occurrence of clinically relevant pathogens for which only limited therapeutic options are available.

A final aspect that should be noted in this context is that in a recent study Rossi et al.⁵⁹ reported vancomycin resistance transmission between MRSA strains isolated from blood cultures.

The most effective antibiotics in MRSA and MRSE isolates were daptomycin, linezolid and vancomycin. The significant and rising trend in the number of MRSA isolates may be an indication of the need to redefine existing strategies to minimize antimicrobial resistance. It is necessary to periodically conduct surveillance studies in all hospitals in the country to achieve effective control strategies of multi-drug-resistant infections and to reduce antimicrobial resistance rates.