In Vitro Susceptibility of Mycobacterium ulcerans Isolates to Selected Antimicrobials

Management of Mycobacterium ulcerans infection (Buruli ulcer) in Australia: consensus statement

INTRODUCTION

Buruli ulcer, caused by Mycobacterium ulcerans, is increasing in incidence and spreading to new areas in southeast Australia. With increasing experience and emerging evidence, this consensus statement considers contemporary data to provide up-to-date recommendations to clinicians who may encounter this disease. The emergence of Buruli ulcer in previously non-endemic areas highlights the importance of increasing clinician and community awareness of this disease. Main recommendations and changes in management as a result of this consensus statement: Buruli ulcer is a notifiable disease in Victoria, the Northern Territory and Queensland. Cases identified in other states or territories should be discussed with relevant health authorities. We call for Buruli ulcer to be made nationally notifiable to monitor for its potential emergence in non-endemic regions. Diagnosis using polymerase chain reaction is sensitive and specific if performed correctly; a dry swab under the undermined edge of ulcers or a tissue sample via punch biopsy if the lesion is not ulcerated is recommended. If swabs are incorrectly performed or performed on non-ulcerated skin, they can give a false negative result. There is high quality evidence to support treatment of eight weeks' duration using rifampicin-based dual oral antibiotic therapy, in combination with clarithromycin or a fluoroquinolone; relapse is very rare but can occur in people with risk factors. There is emerging evidence for shorter durations of treatment (six weeks) in individuals with small lesions who are at low risk of relapse and in those who have undergone surgical excision of the lesion (four weeks). Patients should be warned that ulcers typically enlarge with antibiotic treatment, will not have healed by completion of antibiotics, and take a median of four to five months to heal. Surgical management is usually not required, but may be beneficial to reduce healing times, avoid or reduce the duration of antibiotics, and manage paradoxical reactions. Early identification and treatment of paradoxical reactions is important, as they are associated with increased tissue necrosis and delayed wound healing. Good wound care is critical in successful treatment of Buruli ulcer, as it enhances healing and prevents secondary bacterial infection. Compared with adults, children have a higher proportion of non-ulcerative and severe lesions, are less likely to experience adverse antibiotic effects, but have higher rates of paradoxical reactions; specialist referral is recommended.

Amoxicillin/clavulanate in combination with rifampicin/clarithromycin is bactericidal against Mycobacterium ulcerans

BACKGROUND

Buruli ulcer (BU) is a skin neglected tropical disease (NTD) caused by Mycobacterium ulcerans. WHO-recommended treatment requires 8-weeks of daily rifampicin (RIF) and clarithromycin (CLA) with wound care. Treatment compliance may be challenging due to socioeconomic determinants. Previous minimum Inhibitory Concentration and checkerboard assays showed that amoxicillin/clavulanate (AMX/CLV) combined with RIF+CLA were synergistic against M. ulcerans. However, in vitro time kill assays (TKA) are a better approach to understand the antimicrobial activity of a drug over time. Colony forming units (CFU) enumeration is the in vitro reference method to measure bacterial load, although this is a time-consuming method due to the slow growth of M. ulcerans. The aim of this study was to assess the in vitro activity of RIF, CLA and AMX/CLV combinations against M. ulcerans clinical isolates by TKA, while comparing four methodologies: CFU enumeration, luminescence by relative light unit (RLU) and optical density (at 600 nm) measurements, and 16S rRNA/IS2404 genes quantification.

METHODOLOGY/PRINCIPAL FINDINGS

TKA of RIF, CLA and AMX/CLV alone and in combination were performed against different M. ulcerans clinical isolates. Bacterial loads were quantified with different methodologies after 1, 3, 7, 10, 14, 21 and 28 days of treatment. RIF+AMX/CLV and the triple RIF+CLA+AMX/CLV combinations were bactericidal and more effective in vitro than the currently used RIF+CLA combination to treat BU. All methodologies except IS2404 quantitative PCR provided similar results with a good correlation with CFU enumeration. Measuring luminescence (RLU) was the most cost-effective methodology to quantify M. ulcerans bacterial loads in in vitro TKA.

CONCLUSIONS/SIGNIFICANCE

Our study suggests that alternative and faster TKA methodologies can be used in BU research instead of the cumbersome CFU quantification method. These results provide an in vitro microbiological support to of the BLMs4BU clinical trial (NCT05169554, PACTR202209521256638) to shorten BU treatment.