The five most common gram-negative bacteria responsible for BSIs in the present study were E. coli, K. pneumoniae, P. aeruginosa, A. baumannii and S. maltophilia. E. coli BSIs were more prevalent in the community setting. Whether community or hospital-acquired, they had a relatively high rate of 3GCR resistance, while carbapenem resistance was seen mainly in in-patients and was usually associated with higher mortality.

Our findings matched other BSI studies when comparing our data with the literature. Data on BSI organisms collected from over 200 medical centers in 45 nations showed the prevalent organisms were Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii between 1997 and 2016, with E. coli supplanting S. aureus as of 2005 [11]. Simultaneously, other gram-negative bacteria such as K. pneumonia, P. aeruginosa and A. baumannii also became prominent causative agents in BSI [12]. Similar results were seen in the Far East, Sub-Saharan Africa and Europe [13,14,15,16]. Acinetobacter bacteremia in Lebanon is far more prevalent than in Europe, China and Japan [17]. A similar trend is also seen in South Korea [18]. This is mainly related to outbreaks in specific regions, hospitals and specific intensive care units (ICUs), and they are usually related to less effective infection control measures, highlighting the importance of prevention in the control of Acinetobacter spread [19]. Regionally, a similar trend was seen in Qatar in 2019 [20], but the data from Saudi Arabia in 2015 shows a higher prevalence of Klebsiella (21%), Acinetobacter (15.6%), Stenotrophomonas, Proteus and Serratia [21]. Figure 6 shows the different organisms involved in gram-negative bacteremias across different countries.

Percentage of each organism’s involvement in total Gram-negative bacteremias across different countries

Most gram-negative pathogens, with the main exceptions of E. coli and Salmonella, were more prevalent in the hospital than in the community setting in the present study. Proportions vary among studies and regions; E. coli was the most common species isolated from community-acquired cases, and the healthcare-associated bacteremia in the SENTRY study was responsible for 26% and 15.6% of the cases, respectively [11]. K. pneumoniae was the second most prevalent gram-negative species for both community- and hospital-acquired bacteremia, followed by P. aeruginosa. A. baumannii was also a frequent cause of hospital-acquired bacteremia, accounting for 3.2% of cases [11]. In addition, a study of over one thousand hospitalized patients with BSIs at or during admission in northern Italy found that E. coli, K. pneumoniae, and P. aeruginosa were the most prominent gram-negative BSIs, whether community or hospital-associated [1]. Older age and UTIs are known risk factors for E. coli BSIs, which could explain the predominance of E. coli BSIs in the community setting. In Lebanon, the prominent elderly population struggles with access to adequate healthcare and proper follow-up. There is also increasing antimicrobial resistance to commonly prescribed drugs and inconsistent infection control practices among healthcare centers [22].

As the most common gastrointestinal bacteria, E. coli and K. pneumoniae are the agents most commonly involved in UTIs, possibly explaining their high prevalence in Gram-Negative Bacilli Bacteremias (GNBBs) [23]. BSIs secondary to UTIs are a rising threat worldwide. In the UK, Lishman et al. found that urogenital infections accounted for over half of all E. coli bacteremia episodes. Other sources of infection included biliary (11–27%) and other intra-abdominal infections (4–48%) [24].

In Lebanon, similar to data from the Middle East and North Africa (MENA) [25], UTI remains one of the major driving forces of BSIs and antimicrobial resistance (particularly 3GCR and CRE). The increased prevalence of UTIs with poor access to proper prevention and treatment measures has exacerbated local bacterial resistance [26]. The proportion of BSIs attributed to UTIs in the present study (32%) points to the significant burden and rising threat of UTIs, whose treatment is an essential driver for resistance, and the presence of other significant causes behind bacteremia cases.

Collapsed, inadequate or non-maintained infrastructure and water piping and treatment are primary factors behind the spread of GNBs, especially in the developing Arab countries of the Middle East [27]. Water systems are essential in spreading Pseudomonas and enterobacterales in the community or hospital setting [28]. While the enterobacterales come primarily from the gastrointestinal (GI) tract, Pseudomonas species are more water-related pathogens [9]. In a 2021 Canadian study, upper UTIs were the most common source of infection (40%), followed by bloodstream infections of unknown source (24%) and infection of the hepatobiliary tract (12%) [29]. Other causes of gram-negative organisms spread in hospitals include pillows, linen, dispensers, blood pressure cuffs, skin and ultra-filtrate bags [32].

Resistance trends vary worldwide based on a variety of factors, including antibiotic misuse, their use in plantations and livestock, injection drug use, poor infection control measures and inappropriate infrastructure [30, 31]. Natural or man-made disasters also play a role in antibiotic resistance spread [32]. Specific populations and their contemporary problems can majorly influence resistance trends [33]. For example, the ongoing crises in the Arab world, the ensuing economic and political issues, and poor hygiene may have caused a surge in gram-negative bacterial infections and facilitated the spread of resistance genes from nearby countries in the MENA region [27, 34]. Therefore, approaches such as the “One Health Approach” that emphasises the inter-species exchange of resistance and microbiota become important to mitigate these challenges [31]. Lebanon has been affected by such challenges since 2010, fueling the spread of resistant gram-negative bacteria across the Lebanese population [35].

Our data shows a stable pattern of E. coli resistance to fluoroquinolones and 3GCs over the years, maintained at around 60% and 40%, respectively, while there was a steady rise in CRE rates from 0.2 to 13%. This might be explained by the ease of access to over-the-counter antimicrobials maintained over the years and the increased use of carbapenems [36]. This is different to other areas of the world. A Korean study between 2019 and 2020 comparing resistance patterns of E. coli isolated from either the blood or urine of hospitalized patients found that ampicillin/sulbactam resistance was approximately 40% in blood and 45% in urine, whereas 20.0% of blood isolates and 27.5% of urine isolates showed 3GC resistance. The fluoroquinolone resistance rate was 33.8% [37]. In a Canadian study comparing different E. coli subtypes, cefotaxime, ceftazidime, aztreonam, and cefepime resistance rates (78.9%) of specific isolates (ST131) were higher than those of others (0–12% for non-ST131 isolates). Other significant antimicrobial resistance rates for blood vs urine isolates in Korea between 2020 and 2021 were Ciprofloxacin, 30.0% vs 37.5% and Tetracycline, 30.0% vs 35.0%, respectively [35]. In Lebanon, E. coli has a variety of ST genes, with ST131 being the most prevalent, followed by ST10 and ST69 [38]. In our study, the resistance rates for E. coli in urine are comparable to those in primary bacteremia. In addition, fluoroquinolone and 3GC resistance are similar to that in UTIs, whether in the community or hospital-acquired setting.

A Canadian study examining the epidemiology of extra-intestinal pathogenic E. coli between 2019 and 2020 found high resistance rates to most antibiotics, specifically detecting 3GC resistance in 14.3% of isolates and fluoroquinolone resistance in 28.6% of them [29]. There was an increase in E. coli BSI incidence rates in the population area studied from 2006 to 2016, which coincides with increased resistance rates to antimicrobials, most prominently ceftriaxone (4.2-fold increase) and ciprofloxacin (2.4-fold increase). This correlation could explain the rise of resistance in BSIs [29].

Our data regarding enterobacterales resistance is similar to worldwide trends, showing a growing resistance pattern to various antibiotic classes [39]. Of 103 g-negative isolates in a regional Saudi-Arabian study in 2019, 23.3% were 3GCR. Klebsiella pneumoniae and E. coli were reported as major 3GCR bacteria in hospital settings within and outside Saudi Arabia, with varying rates from 20 to 70% [39]. The tendency of such resistance patterns to spread in some geographical regions and across different hospitals is concerning and warrants quick intervention and continuous surveillance to avoid outbreaks. In this study, over 70% of E. coli were resistant to 2nd to 4th generation cephalosporins. Fluoroquinolone resistance was also found to be highly elevated [39].

In the same study, E. coli resistance to carbapenems was below 10%; it was 18% to piperacillin/tazobactam, 5% to nitrofurantoin and 4.3% to amikacin [39]. These findings were similar in other studies in the same region and further afield [14, 40]. This can be explained by the overuse of antibiotics purchased over the counter in Saudi Arabia despite attempts to restrict their use. This highlights the need for more robust implementation of regulations to restrict the prescription of antibiotics in humans and animals [41]. The high CRE rates can also be attributed to outbreaks in a single institution from the studied region. These data are alarming and show the potential for the spread of drug resistance across the MENA region.

Our data are similar to those from Turkey, where, despite local efforts at antimicrobial stewardship [42], there are similar problems with the OTC dispensing of antibiotics and their availability in tablet forms.

Similar to E. coli, our data showed K. pneumoniae maintained a 36.6% resistance rate to 3GCs and a 34% resistance rate to quinolones throughout the study. These bacteria can spread and cause multiple infections, leading to sepsis [43]. In the Saudi-Arabian gram-negative BSI study, around 15% of isolates of K. pneumoniae were 3GCR. All of the CR K. pneumoniae in the study were taken from ICU patients. ICU Klebsiella isolates showed 80% resistance to 3GCs, 60% to carbapenems, 65% to fluoroquinolones and 22.6% to amikacin [39]. This sheds light on the frequent outbreaks of KPC K. pneumoniae in ICU settings, leading to high CR trends.

Our study further shows the steady increase in CRE rates in BSIs, highlighting the ease of transfer of resistance genes. Among the proposed mechanisms is the spread of these genes via plasmids by contact. This emphasizes the need for more robust antimicrobial stewardship and infection control measures [44]. This is especially important in the Arab world, where ineffective or non-antimicrobial stewardship programs continue to be a problem driving AMR [41]. For instance, CRE K. pneumoniae has been well-documented in Gulf countries and is a rising global threat [45]. A Chinese surveillance study spanning 20 years also showed an increase in CRE K. pneumoniae prevalence [14]. In contrast, a Brazilian study of BSIs showed K. pneumoniae as the most common pathogen among enterobacterales, with 3GC resistance rates of 95.6% and CRE rates of 13.6% [46]. Similar to the findings in our study in the ICU setting, antibiotic failure and higher disease severity translated to poorer outcomes in patients with CR Klebsiella pneumoniae [47].

Pseudomonas aeruginosa is a gram-negative aerobic bacterium typically found in intestinal flora [48]. However, this pathogen is a dangerous opportunist that targets critically ill or immune-deficient patients [49]. It is consistently among the top four most common pathogens in hospital-acquired BSIs and the three most common pathogens detected in the ICU [50].

Our study shows that Pseudomonas species have a 74% resistance rate to fluoroquinolones in BSIs and a 68% resistance rate to carbapenems. Regarding global spread, P. aeruginosa resistant to carbapenems has been frequently reported from some of the Levant and North African Arab countries (> 50% resistance) [10]. Metallo-β-lactamase production has been its primary mechanism of carbapenem resistance in Lebanon [51]. The Asia Pacific (17–50%) and Latin America (64.6%) regions also exhibit high rates of carbapenem resistance compared with Europe (0–35.6%) and North America (10.3–19.4%) [52]. The Japan Nosocomial Infections Surveillance (JANIS) 2016 report, compiling data from 1653 facilities, found that the rates of imipenem and meropenem resistance according to CLSI 2012 breakpoints were 12.3% for P. aeruginosa [53]. In China, the average carbapenem resistance rates range between 9 and 24%, while Extended drug-resistant (XDR) P. aeruginosa proportions were between 1 and 8% [54, 55]. This shows that the resistance trends of Pseudomonas tend to be regional, dictating a possible spread of resistance patterns across countries nearby.

Carbapenemases, porin channel manipulation, and efflux pumps all contribute to the increasing challenges when treating P. aeruginosa [52, 56]. The different resistance mechanisms it possesses give it a versatile pattern of resistance [57]. Its natural reservoir being water makes it easy to infiltrate communities with poor water infrastructure. Its colonization of water supplies in hospitals makes it an exceptionally successful hospital-acquired pathogen [58]. Globally, different clones predominate in each region [59]. ST235, ST654 and ST233 are the most prevalent strains in the MENA region [60]. Genotypic analyses and genome-wide virulence profiling were done in Lebanon, where multiple drug-resistance genes were found, especially in ST235. Resistance mechanisms were mostly enzymatic, efflux pumps and biofilm-producing genes [61, 62]. Porin regulation seems to be slower than other resistance mechanisms, often taking more time to develop after prolonged periods of antibiotic exposure. This makes it a significant mechanism in chronic infections needing long courses of treatment, leading to a poorer response to treatment [63]. Thus, antimicrobial stewardship and adequately treated water systems are essential in controlling resistant Pseudomonas.

Acinetobacter baumannii is an increasingly concerning gram-negative bacterium mainly responsible for hospital-acquired BSIs [11]. Its non-motile characteristic makes it exceptionally resilient, able to recur several months after cleaning [64]. Its prevalence and resistance profiles depend highly on the regional and local institutional epidemiology. It can vary depending on differences in infection control measures [65]. Outbreaks of MDR A. baumannii have been reported in countries during economic crises, which might explain their increased burden in recent years in Lebanon [66]. In addition, MDR A. baumanii outbreaks have also been reported in war and conflict-affected areas. This possibly added to the burden in Lebanon, where ongoing conflicts were occurring during the study period [66]. Most A. baumannii infections (75%) and antimicrobial resistance (86%) are found in the healthcare setting [67]. Pneumonia and UTIs are common sources of A. baumannii infections in the community setting. At the same time, invasive medical procedures and more extended hospital stays are potential sources of infection in the hospital setting [68, 69]. Its nasal colonization rates were between 72 and 90% in Taiwan and 63% in the USA in long-term inpatients [70]. A. baumannii’s ability to create biofilms to survive on most hospital equipment and expanding resistance profile highlights the need for urgent infection control measures to control its spread [71].

XDR A. baumannii may arise through various mechanisms similar to those of P. aeruginosa. In the last few years, A. baumannii has also been labelled a difficult-to-treat (DTR) organism, resistant to all first-line antimicrobial dr