Pseudomonas aeruginosa is an opportunistic pathogen renowned for its arsenal of virulence factors and its extraordinary capacity to adapt to hostile environments. It is the third most common Gram-negative bacillus isolated from intensive care units globally, and it disproportionately strikes immunocompromised patients, including those with cystic fibrosis, chronic obstructive pulmonary disease, and bronchiectasis. The bacterium deploys quorum sensing activation and biofilm formation to colonise the lungs and persist on moist hospital equipment such as mechanical ventilators, where it resists certain antiseptics and causes ventilator-associated pneumonia, a leading cause of mortality in critically ill patients. Its lipopolysaccharide chain mediates attachment to host cells while its flagellum drives motility, and it excretes exotoxins that disrupt host cell functions alongside enzymes that facilitate tissue invasion and immune suppression. Treatment difficulties stem principally from efflux pumps that expel antibiotics from the bacterial cell and enzymes that inactivate them, and the pathogen acquires further resistance through horizontal gene transfer and mutation. Multidrug-resistant strains are defined as those resistant to three or more categories of antipseudomonal agents, while extensively drug-resistant strains remain susceptible to only two or fewer categories, and the difficult-to-treat designation introduced in 2018 refers to strains resistant to nearly all antipseudomonal agents including all first-line options.

The study’s methodology was straightforward but rigorous. Researchers analysed de-identified secondary data from the main hub of a private laboratory group with 250 laboratories throughout Gauteng province, capturing results in an Excel spreadsheet after ethical approval was granted in February 2025. Isolates were identified using Gram staining, colony morphology, rapid bench tests, and MALDI-TOF mass spectrometry, while antimicrobial susceptibility testing employed the Kirby-Bauer disk diffusion method and the automated VITEK-2 system. All zone of inhibition diameters and breakpoints were interpreted according to Clinical and Laboratory Standards Institute guidelines, with the laboratory updating its breakpoints annually across the M100 editions from Ed29 in 2019 through Ed33 in 2023. Notably, in 2023 the CLSI ceased to recommend gentamicin as a treatment option for Pseudomonas aeruginosa, and the laboratory stopped reporting susceptibility results for that antibiotic. Each requisition number was treated as an individual sample, and isolates were classified as multidrug-resistant if they showed resistance to four or more antibiotics from the testing panel, ensuring that three or more antipseudomonal categories were represented.

The demographic picture that emerged is striking. Among the 16,167 positive samples, the predominant age group was patients over 60 years, accounting for 50.8 percent of isolates with a mean age of 74 years, while the 11-to-20-year age group contributed the fewest isolates. This concentration in the elderly aligns with a 2023 Saudi Arabian study reporting that the age group most affected by multidrug-resistant Pseudomonas aeruginosa is 50 years and older, and it resonates with recent Gauteng research on urinary samples from long-term care facilities suggesting that higher antimicrobial resistance rates in older patients may stem from empirical urinary tract infection treatment. The clinical specimen distribution was equally revealing: urine samples yielded the highest proportion of isolates at 30.7 percent, respiratory samples followed at 28.7 percent, and tissue samples accounted for 21.1 percent, while ear swabs at 0.9 percent and fluids at 2.4 percent contributed minimally. A Kuwaiti study reported similar findings, with respiratory samples at 42.5 percent and urine at 20.3 percent, whereas other international studies have found ESKAPE pathogens more commonly in urine and blood cultures.

When researchers compared sensitive versus multidrug-resistant isolates across each year, the chi-square result of 184.704 with a p-value below 0.001 indicated a high level of statistical significance over time, confirming that the year of collection was genuinely associated with changes in susceptibility. The cumulative rise in multidrug-resistant isolates amounted to a 2.9 percent increase from 2019 to 2023, with a pronounced surge between 2019 and 2021. Blood culture samples carried the highest proportion of multidrug-resistant isolates in 2019 at 32.3 percent, 2020 at 31.5 percent, and 2023 at 33.3 percent, with a 7.3 percent increase between 2019 and 2022 followed by a 6.3 percent decline in 2023. Pus fluid samples led in 2021 and 2022, peaking at 41.3 percent multidrug resistance in 2022. Respiratory samples showed a 6 percent surge in multidrug-resistant isolates from 2020 to 2021 and an overall 5.5 percent increase across the five-year period. The researchers suggest that the blood culture increase could reflect prolonged intensive care unit stays during South Africa’s COVID-19 waves, when glucocorticoids used to treat severe cases have been independently associated with the development of bacteraemia in Japanese studies.

The antibiotic-level trends are perhaps the most alarming technical finding. In 2019, only three antibiotics—ciprofloxacin, imipenem, and meropenem—showed resistance above 20 percent. By 2021 that number had grown to five, adding doripenem and ceftazidime-avibactam. In 2022, six antibiotics exceeded the 20 percent threshold, adding cefepime and ceftolozane-tazobactam. By 2023, seven antibiotics showed resistance above 20 percent, adding aztreonam. Carbapenems consistently presented the highest resistance rates, with imipenem and meropenem exceeding 20 percent in all five years and doripenem crossing that threshold from 2021 onward. Resistance to amikacin rose by 2.4 percent, tobramycin by 0.9 percent, ceftazidime by 3.8 percent, cefepime by 3.3 percent, piperacillin-tazobactam by 3.9 percent, imipenem by 3.6 percent, meropenem by 2.3 percent, and doripenem by 6.1 percent over the study period. Gentamicin resistance declined by 4.5 percent, though the researchers attribute this partly to reduced reporting following EUCAST discouragement in 2020 and CLSI guideline updates in 2023 rather than a true biological reversal. Ciprofloxacin, the only fluoroquinolone tested, declined by 4.7 percent, a decrease the researchers suggest could reflect reduced use and the possibility of resistance reversal in antibiotic-free environments where selective pressure for nfxB gene mutations driving efflux pump overexpression is absent.

Perhaps most concerning is the resistance observed against the newest antipseudomonal agents. Ceftazidime-avibactam was tested against 666 multidrug-resistant isolates, of which 454 showed resistance, yielding a rate of 71.7 percent in 2021, 66.7 percent in 2022, and 63.2 percent in 2023. Ceftolozane-tazobactam was tested against 559 multidrug-resistant isolates, with 379 showing resistance, producing rates of 73.4 percent in 2022 and 63.4 percent in 2023. Aztreonam, tested against only 14 multidrug-resistant isolates with 5 showing resistance, registered 33.3 percent in 2023. The researchers caution that because these newer antibiotics were largely restricted to multidrug-resistant isolates and registered in South Africa only towards the end of the study period, the generalisability of these findings to the broader isolate population is limited. Colistin, a last-line polymyxin reserved for extensively drug-resistant infections, showed a 7.8 percent resistance increase from 2019 to 2020 followed by a 10 percent decline from 2020 to 2021, and its resistance presence is troubling for critically ill patients because chromosomal mutations or plasma-mediated spread of the mcr gene can eliminate this final option.

Sample-type analysis revealed where resistance concentrates most heavily. Respiratory samples presented the highest prevalence of carbapenem resistance at 42.1 percent and beta-lactam resistance at 37.7 percent, with urine samples second at 23.8 percent and 28.9 percent respectively, and tissue samples third at 14.1 percent and 14.6 percent. For aminoglycosides, urine samples led at 38.8 percent, respiratory samples followed at 30.6 percent, and tissue samples registered 12.8 percent. This pattern correlates with Iranian findings reporting tobramycin resistance of 45 percent and amikacin resistance of 43 percent among aminoglycoside-resistant isolates. The respiratory concentration is particularly worrying given that a 2024 Norwegian cohort study found that 70 percent of hospitalised patients with respiratory viral infections and positive PCR tests but no evidence of bacterial infection were nonetheless administered antibiotic therapy, and a 2024 systematic review concluded that the COVID-19 pandemic influenced Pseudomonas aeruginosa resistance patterns through intense antibiotic use. Patients with severe respiratory viral infections frequently required intensive care admission and invasive mechanical ventilation for prolonged periods, creating ideal conditions for ventilator-associated pneumonia and the spread of resistant strains within hospital settings.

The study’s authors conclude that antibiotic stewardship, together with infection control, plays a decisive role in preventing the development and spread of resistant strains. Stewardship programmes aim to reduce misuse and overuse by promoting narrow-spectrum antibiotics and discouraging empirical broad-spectrum therapy, which can accelerate resistance before culture results identify the true pathogen. World Health Organisation infection control recommendations—including rigorous hand hygiene, glove and gown use, patient isolation, environmental disinfection, and surveillance—are particularly critical in intensive care units prone to nosocomial infections. The researchers acknowledge limitations: the retrospective data came from a single private laboratory and may represent only the private sector epidemiology rather than the public sector, patient clinical records were absent so previous antibiotic exposure could not be assessed, and de-identified datasets made it impossible to determine whether multiple requisition numbers belonged to the same patient, especially among intensive care patients who may submit repeated samples. Despite these constraints, the findings provide crucial insight into the antimicrobial patterns of a WHO priority pathogen in Johannesburg and underscore the urgent demand for new antipseudomonal drugs and expanded surveillance of Pseudomonas aeruginosa and other ESKAPE pathogens in the clinical setting, contributing to the global antimicrobial resistance discourse and supporting future treatment decisions.

Subject of Research: Carbapenem Resistance Climbs in Johannesburg Pseudomonas aeruginosa Isolates

Article Title: ESKAPE pathogen alert: resistance trends in Pseudomonas aeruginosa from clinical isolates in Johannesburg (2019–2023)

Article References: Chaggan, S., & Mieta, S. I. K. (2026). ESKAPE pathogen alert: resistance trends in Pseudomonas aeruginosa from clinical isolates in Johannesburg (2019–2023). Frontiers in Bacteriology, 5, Article 1869638. https://doi.org/10.3389/fbrio.2026.1869638

Image Credits: AI Generated

DOI: 10.3389/fbrio.2026.1869638

Keywords: Pseudomonas aeruginosa, antimicrobial resistance, ESKAPE pathogens, carbapenem resistance, Johannesburg, South Africa, multidrug-resistant, nosocomial infections, antibiotic stewardship, infection control, COVID-19 pandemic, clinical isolates