Haemophilus parainfluenzae is a Gram-negative coccobacillus of the microbiota of the upper respiratory tract. Evidence has recently been found of its role as a pathogen causing non-gonococcal urethritis (NGU), although the pathogenicity is difficult to assess, as it can also be isolated in asymptomatic patients or along with other pathogens.1,2 The emergence of multidrug resistant (MDR) strains of H. parainfluenzae has recently been documented in different countries in Europe.3–6 As the empirical treatment for urethritis is a third-generation cephalosporin with or without a macrolide, these drugs would not be effective against MDR H. parainfluenzae.
We present the isolates of H. parainfluenzae obtained from genital samples analysed at Hospital Universitario de Navarra from 2016 to March 2022.
The isolates were identified using MALDI-TOF (Bruker Daltonik, Germany) and the antibiogram was performed using the disc-plate method on Haemophilus Test Medium agar (Becton Dickinson, United States) with 24h incubation at 37°C in an atmosphere with 5% of CO2, studying the sensitivity to ampicillin, amoxicillin/clavulanic acid, cefuroxime, cefotaxime, nalidixic acid, ciprofloxacin and cotrimoxazole. We followed the cut-off points established by EUCAST in 2021.7 Beta-lactamase activity was studied using a nitrocefin disc (cefinase, BD, USA) and/or molecular detection of TEM/ROB enzymes. An MDR strain was defined as resistant to three or more classes of antibiotics.8
During the study period, 443 isolates of H. parainfluenzae were obtained from genital samples, with or without clinical significance. The mean age of the patients was 34 years (SD: 13 years), and 345 (77.8%) were male.
In the susceptibility study, 13 (2.2%) MDR strains were identified, all in males with a mean age of 32 (SD: 5 years). Table 1 shows how MDR H. parainfluenzae has evolved in Navarra.
We found a significant increase in resistance, from not isolating any resistant strains to having 18% MDR strains in seven years. All MDR H. parainfluenzae strains were resistant to ampicillin, amoxicillin/clavulanic acid and cefuroxime. One strain also showed resistance to quinolones, five strains to cotrimoxazole, and five were resistant to all the antibiotics studied. Two types of resistance mechanisms to beta-lactams have been described in this genus: a) enzymatic hydrolysis by TEM-type beta-lactamases, or less frequently ROB-types, a phenotype known as beta-lactamase-positive ampicillin-resistant (BLPAR); and b) resistance due to modifications in penicillin-binding proteins (PBP), beta-lactamase-negative ampicillin-resistant (BLNAR) phenotype.3,9 In addition, the two mechanisms can coexist in the same strain, also decreasing susceptibility to amoxicillin/clavulanic acid and second-generation cephalosporins. Strains with double resistance mechanism can be detected by positive cefinase test and amoxicillin/clavulanic acid halo diameter ≤12mm.7 In our study, the resistance to beta-lactams of MDR H. parainfluenzae was due in 58.3% to alteration of PBP plus beta-lactamases (66.7% of the cases due to production of TEM) and in 41.7% to PBP alteration only.
In conclusion, as has been reported elsewhere, we have found that in our hospital there has been an increase in MDR H. parainfluenzae in genital samples in recent years, mainly in males aged 30–40.4–6 This increase may be due to the increase in the number of genital samples taken following the opening of the sexually transmitted infection clinic in our department, and the fact that the microorganism is actively looked for in cultures, identifying it and performing a susceptibility study for proper surveillance.
The potential role of H. parainfluenzae in urethritis needs to be taken into account, as empirical treatment in monotherapy with ceftriaxone might not be effective against MDR isolates. Therefore, when a patient has symptoms consistent with urethritis, in addition to starting empirical antibiotic therapy, we have to take samples for culture, identify H. parainfluenzae as the probable cause of NGU and determine the susceptibility profile, in order to closely monitor the emergence of resistant H. parainfluenzae isolates.
