Poster Presentation 25th International Pathogenic Neisseria Conference 2026

Background:

Beta-lactam antibiotics are used to treat invasive meningococcal disease, while ciprofloxacin is used for chemoprophylaxis to reduce oropharyngeal meningococcal carriage. The emergence of ciprofloxacin resistant (CIP^R) and penicillin resistant (PEN^R) isolates of N. meningitidis is a public health concern, as this reduces the effectiveness of treatment and outbreak control measures. We investigated the prevalence of CIP^R and PEN^R meningococcal carriage isolates from Western Australia (WA).

Methods:
209 carriage isolates were collected between 2017 and 2025. They were screened for susceptibility to PEN and CIP using agar dilution and ETEST^® (BioMérieux). CIP^R and PEN^R isolates were whole genome sequenced (WGS) using the Illumina MiSeq platform. Comparative genomic analyses were performed using PubMLST Genome Comparator and Geneious Prime (v2023.2.1). Maximum-likelihood phylogenies were inferred using IQ-TREE and annotated in iTOL.

Results:
Agar dilution testing revealed decreased susceptibility to PEN in 23.9% isolates (50/209), decreased susceptibility to CIP in 6.2% (13/209) and a further 2.9% isolates (6/209) exhibited dual decreased susceptibility. ETEST^® confirmed one PEN^R isolate (MIC 0.50 µg/mL) and high-level CIP^R in 8.6% of the isolates (18/209; MIC range 0.125 - 0.38 µg/mL). WGS of selected isolates revealed they belonged to ST-11563 (capsule null locus), ST-4977 (cc11, MenW), ST-11026 (cc32, non-groupable), and ST-5662 (cc 4821, MenB). ClustalW alignment of gyrA alleles confirmed the GyrA T91I substitution. The PEN^R isolate was closely related to the WA MenW:cc11 outbreak lineage (2012-2020) and possessed the penA_9 allele. CIP^R isolates were distributed across multiple lineages. One isolate belonged to ST-11026, a highly clonal lineage arising in Japan in 2014. A second isolate clustered with ST-5662, originally described in China in 2007. The remaining four isolates belonged to ST-11563, a genetically variable cluster associated with international carriage studies. All 38 international isolates are predicted to be CIP^R. The Australia isolates are the earliest recorded in PubMLST.

Conclusion:
PEN^R was rarer than CIP^R in meningococcal carriage isolates in this time period. Diverse international genetic lineages possessed GyrA T91I, implying international travel-associated introductions of ST-5662 and ST-11026 into Perth. ST-11563 has likely evolved locally in response to the continued use of CIP prophylaxis of close contacts of IMD patients.