Resistance Markers Define the European ST10042 Lineage
The antimicrobial profile of Campylobacter coli sequence type 10042 leaves little room for therapeutic optimism. Across the European isolate set, ciprofloxacin resistance was universal, tetracycline resistance was nearly universal, and ampicillin resistance was fixed in the phenotype data from Portuguese isolates. The pattern is not random. It reflects a lineage that has accumulated the exact resistance determinants most likely to blunt first-line treatment choices for campylobacteriosis and to erode the usefulness of drugs often reached for when diarrhea becomes severe enough to prompt care.
The strongest signal sits in the fluoroquinolone class. Every one of the 217 isolates carried the GyrA Thr86Ile substitution, matching the phenotype in which all 98 Portuguese isolates were resistant to ciprofloxacin. In practical terms, that mutation alters DNA gyrase in a way that prevents fluoroquinolones from binding effectively. A lineage carrying that determinant at complete frequency is already well beyond the point of isolated resistance events; it behaves like a stable resistant clone.
Tetracycline resistance followed a similar script. The phenotype showed resistance in all 98 Portuguese isolates, while the genotypic data found tet(32/O/32) in 213 of 217 isolates and tet(O) in 4 of 217. In a pathogen that moves through the food chain and into humans through repeated animal and environmental interfaces, this kind of resistance suggests sustained selective pressure rather than a one-off outbreak strain. The lineage appears equipped to survive in settings where tetracyclines have long been used in veterinary medicine and where resistance can become embedded in circulating animal reservoirs.
Beta-Lactam Resistance Was Dominated by OXA-61-Associated Mechanisms
Resistance to beta-lactams was also extensive. In the Portuguese phenotype set, ampicillin resistance reached 98 of 98 isolates, with MIC values above 256 mg/L. For amoxicillin/clavulanic acid, 95 of 98 isolates were resistant, 2 were susceptible, and 1 lacked the phenotype result. On the genetic side, bla OXA-61 appeared in 216 of 217 isolates, and nearly all of those carried the promoter variant -57G>T. The combination is important because it explains why beta-lactam exposure may fail even when clavulanic acid is added to inhibit beta-lactamase activity.
A single lineage repeatedly pairing the enzyme with a promoter change points to more than enzyme presence alone; it indicates increased expression of the resistance mechanism. In bedside terms, that matters because beta-lactams are often selected when clinicians want an oral option for enteric bacterial illness with a presumptive bacterial cause. Here, the phenotype says that assumption would frequently be wrong. The lineage is not merely harboring resistance genes; it is expressing them in a way that preserves the organism’s survival against a common class of antibiotics.
There were a few outliers. One isolate carried a more complex amoxicillin/clavulanic acid resistance profile involving bla OXA-61, the promoter change, a -69delA deletion, and porA 35. Another 8 of 98 Portuguese isolates had amoxicillin/clavulanic acid MICs at 8/0.094 mg/L and were interpreted as susceptible. Even with those exceptions, the broader pattern remains firmly resistant.
Macrolides Retained Partial Activity, But Only on a Narrow Margin
Macrolides were the one class in which the lineage did not achieve near-total resistance. Among the Portuguese isolates, 96 of 98 were susceptible to erythromycin, and 2 were resistant. Genotypically, macrolide resistance determinants were uncommon, but they were present: 6 of 217 isolates carried 23S rRNA A2075G, and one each carried A2074G and A2074C. The dominance of susceptibility here is clinically meaningful because erythromycin remains one of the few oral options that can still work against campylobacteriosis when susceptibility is preserved.
The margin is narrow, though. A lineage can remain broadly macrolide-susceptible and still contain the mutational seeds for future treatment failure. Ribosomal mutations in 23S rRNA directly impair macrolide binding at the bacterial ribosome, and even a small reservoir of those variants creates a warning sign for future expansion under antibiotic pressure. In a pathogen already resistant to fluoroquinolones and tetracyclines, preserving macrolide activity becomes a public health priority rather than a reassuring detail.
Ertapenem Susceptibility Does Not Translate Into a Practical Oral Option
Ertapenem testing showed decreased susceptibility in 96 of 98 Portuguese isolates, with only 2 appearing susceptible. Genotypically, 213 of 217 isolates carried porA 35/1006/4736, while 4 lacked the determinant. The presence of altered porin-associated markers is consistent with reduced carbapenem susceptibility, although carbapenems are not typical frontline therapy for campylobacteriosis and are not a realistic routine outpatient solution. The finding matters less as a direct treatment roadmap than as a sign of how far this lineage has moved in the direction of multidrug resistance.
The picture becomes even starker when the entire resistance profile is considered together. Fluoroquinolones are out. Tetracyclines are out. Ampicillin is out. Amoxicillin/clavulanic acid performs poorly. Macrolides remain the main class with substantial activity, but only because resistance has not yet become dominant. That kind of asymmetry usually marks a lineage that has already adapted to multiple selective environments, with human and animal exposure likely reinforcing one another.
Surveillance Has to Track Human and Veterinary Circuits at the Same Time
The table makes one point unmistakable: this is not a resistance problem confined to a single clinical ward or a single country. The lineage was sampled across Europe, and the resistance markers were already broadly distributed across the 217-isolate genotypic set. The multisectoral nature of the findings is the real warning. When a foodborne pathogen shows coordinated resistance across classes that matter in both veterinary and human medicine, the reservoir is rarely limited to one chain of transmission.
For clinicians, the consequence is straightforward. Empiric choices for suspected campylobacteriosis cannot assume fluoroquinolone activity in this lineage, and tetracycline is unlikely to rescue treatment. For microbiology and public health teams, the more urgent task is to follow the lineage through animal sources, food production pathways, and human cases before macrolide resistance becomes more common. Once a lineage already carrying GyrA Thr86Ile, tet(O) family determinants, and bla OXA-61 gains broader macrolide escape, treatment options narrow sharply.
The table leaves one final concern unresolved: how much this lineage has already spread beyond the isolates captured in the European dataset, and how quickly the rare erythromycin-resistant subpopulation may expand under antibiotic pressure. The genetic ingredients are in place, and the phenotypes show that the organism already knows how to survive across several major drug classes.