After many years in which only isolated cases of cutaneous diphtheria occurred and almost three decades without a single case of respiratory diphtheria, increased cases of diphtheria caused by Corynebacterium diphtheriae(C. diphtheriae) occurred in 2022. In addition to Austria, this also affected other countries in the EU/EEA.

At the National Reference Centre for Diphtheria - Laboratory, all strains were tested for susceptibility to penicillin, linezolid, clindamycin, rifampicin and doxycycline and, for some strains, also to amoxicillin, trimethoprim-sulfamethoxazole, ciprofloxacin, vancomycin, tetracycline, erythromycin, azithromycin and clarithromycin using the E-test (Biomerieux, Marcy-l'Étoile, France). Until the pre-publication of the clinical breakpoints of the European Committee on Antimicrobial Susceptibility Testing (EUCAST) version 13.0 in autumn 2022, there were no valid species-specific breakpoints for C. diphtheriae and C. ulcerans. As a result, the interpretation was made according to the recommendations of the WHO Laboratory Manual 2021. There are still no clinical breakpoints for vancomycin, clarithromycin and azithromycin.

For a better overview of the current resistance situation, the susceptibility was analysed retrospectively with interpretation according to the currently valid clinical breakpoints of EUCAST version 13.0. Due to the great importance, the analysis was carried out for all toxin-producing C. diphtheriae isolates. No phenotypic resistance to penicillin was detected in any of the 71 C. diphtheriae isolates; all isolates showed sensitivity to increased exposure according to EUCAST. In phenotypic resistance testing to amoxicillin, isolates also showed sensitivity at standard doses. Phenotypic resistance to erythromycin was detected in a total of six (8.5 %) isolates. High resistance rates were also observed to clindamycin (23.9 %), doxycycline (32.4 %), tetracycline (35.2 %), ciprofloxacin (26.8 %) and trimethoprim-sulfamethoxazole (66.2 %).

A Europe-wide consortium, including the National Reference Centre for Diphtheria Laboratory, was set up to investigate and contain the outbreak. An analysis of all diphtheria cases in the period from 1 January to 30 November 2022, which were associated with facilities for migrants, was carried out. The analysis of genetic relatedness and antibiotic susceptibility testing showed a simultaneous occurrence of three different sequence types (ST377, ST384 and ST574) in several countries with closely related isolates. Within the ST377 sequence type, a subcluster also emerged that showed a worrying antimicrobial resistance profile to first-line antibiotics (macrolides) used in the treatment of diphtheria. Strains of this subcluster were also identified in Austria.

Further monitoring of the resistance situation, especially macrolide resistance, is particularly important in order to be able to react quickly to an increase in resistance with modified treatment recommendations.

The National Reference Centre for Gonococci operates a national gonococcal surveillance system based on a sentinel system, which focuses on monitoring antimicrobial resistance. The reference centre is therefore part of the "European Gonococcal Antimicrobial Surveillance Programme" (Euro-GASP).

Neisseria gonorrhoeae(N. gonorrhoeae) has developed resistance to all classes of antibiotics used for treatment since the introduction of antimicrobial treatment in the 1930s. Multi-resistant strains are a major public health problem worldwide.

According to current national and international guidelines, ceftriaxone or cefixime are currently used as monotherapy or in combination with azithromycin for the calculated treatment of gonococcal infections. An important component of gonococcal surveillance systems is therefore the monitoring of ceftriaxone and azithromycin resistance in particular.

In 2024, all isolates tested at the reference centre for gonococci were sensitive to ceftriaxone in vitro. In 2022, there was one ceftriaxone-resistant isolate among the strains tested at the National Reference Centre for Gonococci for the first time. This isolate was also found to have high-level resistance to azithromycin and resistance to cefixime, ciprofloxacin and tetracycline (extensively drug-resistant, XDR). No MDR/XDR isolates occurred in 2023 and 2024.

None of the isolates were found to be resistant to cefixime in 2024. This was also the case in 2023, three resistant isolates were detected in 2022, one resistant isolate in 2021 and no cefixime-resistant isolates were detected in 2020. In the previous years, higher resistance rates to cefixime were observed at 2% (2019), 3% (2018) and 4% (2017 and 2016) respectively.

In 2024, 63.6 % (n = 259) of isolates showed resistance to ciprofloxacin, which was slightly below the level of the last three years (2023: 73 %, 2022: 77 %, 2021: 75 %, 2020: 58 %, 2019: 62 %, 2018: 57 %, 2017: 50 %, 2016: 65 %). Within the resistant isolates, eleven isolates (corresponding to 2.7% of all isolates) showed high-level resistance (MIC ≥ 32) in 2024, which is a decrease compared to the previous year (2023: 4.5%).

A total of 14.5% of isolates (n = 59) had an azithromycin MIC of more than 1 µg/ml, which is an indication of acquired resistance. In 2024, there was thus a slight decline in acquired azithromycin resistance after a stable phase between 2019 and 2020 with a proportion of approx. 14 % of resistant isolates (2023: 22.9 %, 2022: 28 %, 2021: 20 %). In 2017, the resistance rate according to the guidelines valid at the time was still 4% (MIC > 1µg/ml in 2017: 2.8%). Within the resistant isolates, one isolate (corresponding to 0.2 % of all isolates) from 2024 also showed high-level resistance (MIC ≥ 256 µg/ml), which is comparable to the previous year (2023: 1 %).

At 75.4 % (n = 307), the tetracycline resistance rate increased significantly in 2024 compared to previous years (2023: 66.7 %, 2022: 47 %, 2021: 36 %, 2020: 47 %, 2019: 49 %, 2018: 41 %, 2017: 43 %), although this is also related to the change in the breakpoint by EUCAST from version 13.0 in the year 2023.

In 2024, only 5.7 % of isolates (n = 23) showed sensitivity to penicillin at standard doses (2023: 4 %, 2022: 6 %, 2021: 14 %, 2020: 17 %, 2019: 17 %, 2018: 21 %), continuing a downward trend. A total of 74.7 % (n = 304) of the isolates were sensitive to increased penicillin exposure (2023: 77 % 2022: 79 %, 2021: 73 %, 2020: 71 %, 2019: 72 %, 2018: 63 %) and 19.4 % of the isolates (n = 79) had an MIC in the resistant range. Of these, four isolates tested negative using the nitrocefin or cefinase test and 75 isolates tested positive for the presence of a ß-lactamase. Together with one further isolate, which was tested as sensitive at increased exposure and in which a ß-lactamase was also detected, this resulted in an overall resistance rate of 19.7 % of isolates for 2024 (2023: 22 %, 2022: 16 %, 2021: 14 %, 2020: 18 %, 2019: 11 %, 2018: 16 %, 2017: 17 %; 2016: 30 %).

In Austria, all isolated Salmonella strains are sent to the National Reference Centre for Salmonella (NRZS). This applies to Salmonella strains obtained from human medical test material (e.g. faeces, blood, smears, etc.) as well as isolates from veterinary material and food samples as well as feed and environmental samples.

At the NRZS, all isolates undergo serotyping, biochemical differentiation and resistance testing, and antibiotics are selected in accordance with current ECDC guidelines. Based on epidemiological criteria, antibiotics that are not suitable for treatment are also tested. To recognise low-level ciprofloxacin resistance, pefloxacin is used instead of ciprofloxacin in accordance with the EUCAST guidelines.

The treatment of "normal" diarrhoea caused by Salmonella consists primarily of fluid and electrolyte replacement. There is no need for antibiotic therapy as this does not change the severity or duration of the illness or bacterial excretion. However, the use of antibiotics is necessary in severe cases (e.g. bloody diarrhoea), if infants or elderly people are affected or if the illness goes beyond diarrhoea (e.g. joint inflammation, sepsis).

Antibiotic resistance - human isolates

The resistance rates for human Salmonella strains have shifted significantly upwards in Austria in recent years - due to the sharp decline in S. Enteritidis isolates, most of which are fully sensitive.
In 2022, the resistance rates to several antibiotics (ampicillin, sulphonamides, tetracycline) were - in some cases significantly - above 15 % (see table). The main reason for this is the increased occurrence of multi-resistant S. Typhimurium strains. Due to the frequent occurrence of nalidixic acid/low-level ciprofloxacin-resistant S. Enteritidis and S. Infantis isolates, the low-level ciprofloxacin resistance rate was 20.9 %. The proportion of multidrug-resistant isolates (defined as resistance to three or more antibiotic classes) was 17.2 %.

The resistance rates are often also an expression of the frequent occurrence of a particular clone. For example, the unusually high nalidixic acid/low-level ciprofloxacin resistance rates (> 36 %) in 2019 are attributable to an outbreak with more than 300 microbiologically confirmed cases of illness caused by a nalidixic acid/low-level ciprofloxacin-resistant S. Enteritidis strain in Polish eggs.

Table: Proportion of resistance in all first human Salmonella isolates, Austria, 2000 - 2022 (Note: not all antibiotics tested are listed. The number of resistant isolates is given in brackets).

Bacteria of the genus Campylobacter are among the most important pathogens of bacterial diarrhoeal diseases worldwide. Every year, 6,000-7,000 cases of campylobacteriosis are reported in Austria, almost 90 % of all infections are caused by Campylobacter jejuni, the second most common species is Campylobacter coli. As a rule, diarrhoea is self-limiting and does not require antibiotic treatment. Under certain circumstances - such as severe illness or immunosuppression - it may be necessary to administer antibiotics to treat a Campylobacter infection.

Antibiotic resistance in Campylobacter is monitored in humans, in farm animals and in foodstuffs that are important sources of infection, either as part of the EU-wide monitoring of zoonotic agents (Implementing Decision (EU) 2020/1729) or a national surveillance programme of the reference centre. Every year, a certain number of isolates of the two most common species are tested for the antimicrobial agent classes relevant for use in human medicine.

For years, very high to extremely high resistance to fluoroquinolones has been observed in all areas - the proportion of resistant human C. jejuni isolates rose from 52.1 % in 2007 to 80.1 % in 2022 - and high to very high resistance to tetracyclines, and even extremely high resistance to tetracyclines in pigs (Fig. 1 and Fig. 2). For a long time, fluoroquinolones were considered the drug of choice for the treatment of severe Campylobacter infections. Due to the high resistance rates to this drug class in many countries, other drugs such as macrolides (azithromycin) are now the preferred treatment options. While some EU countries have recorded medium to high resistance rates to macrolides in human clinical isolates, a stable low resistance rate has been observed in Austria for years; the 5-year average (2018-2022) for C. jejuni isolated from human clinical samples is 0.2% and for C. coli 3.1% (data from the reference centre, unpublished). No resistance or low resistance rates are generally observed for gentamicin.

In 2022, the proportion of multidrug-resistant C. jejuni and C. coli isolated from patients was 0.7 % and 1.2 % respectively (data reference centre, unpublished). Furthermore, the occurrence of horizontally transmissible high-level macrolide resistance observed in Asia and Europe requires intensified monitoring. In Austria, a resistance determinant responsible for this, erm(B), has so far only been detected in individual patient isolates. The carbapenem resistance described in isolated patient isolates and the indication of a possible increased resistance to ertapenem in isolates from some livestock populations in the EU, which is currently the subject of ongoing investigations, will also require increased attention in the future.

References:

AURES 2020, AURES 2021

Annual Report 2022 of the Reference Centre for Campylobacter

The European Union Summary Report on Antimicrobial Resistance in zoonotic and indicator bacteria from humans, animals and food in 2020/2021 https://doi.org/10.2903/j.efsa.2023.7867