Respiratory sepsis due to Moraxella atlantae: Utility of mass spectrometry to identify rare species

We present the case report of a 77-year-old man with a history of atrial fibrillation, chronic obstructive pulmonary disease (COPD) GOLD stage IV with frequent exacerbations and a former smoker, who was admitted to hospital two weeks previously due to a COPD exacerbation. He was taking acenocoumarol and using a formoterol and glycopyrronium inhaler. The patient attended the emergency department due to fever, dyspnoea and cough with greenish expectoration.

Physical examination revealed a respiratory rate of 28 breaths per minute, blood pressure 100/55 mmHg, heart rate 85 bpm, temperature 38.3 °C, crackles at the base of the right lung and rhonchi in both upper lobes. The rest of the physical examination was normal. The blood test showed haemoglobin 14.4 mg/dl, leukocytes 10.70 × 10³/μl, creatinine 0.71 mg/dl, sodium 128 mEq/l, C-reactive protein 3.6 mg/dl and oxygen blood pressure 65 mmHg. The chest X-ray revealed right basal consolidation. Blood cultures were taken and empirical antibiotic therapy with meropenem was started given the clinical suspicion of respiratory sepsis.

The patient's progression during admission in internal medicine was favourable, with resolution of fever at 48 h. One of the blood cultures was positive after 4 days of incubation. Gram staining was performed which revealed Gram-negative bacilli, and subcultures were produced in standard media.

The colonies were processed by Matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF), which identified Moraxella atlantae (M. atlantae) with a score of 2.1.

Antibiotic therapy with meropenem, which was started in the emergency department, was maintained with good patient progression. After being admitted for 7 days, the patient was discharged home.

Discussion

The genus Moraxella comprises more than 20 species of aerobic, immobile, oxidase-positive Gram-negative cocci.¹M. atlantae is an opportunistic microorganism that is found in human saprophytic flora. It is a very difficult microorganism to find and isolate in blood cultures.² We only found 2 cases in the literature of M. atlantae causing infection, isolated from blood cultures conducted due to fever of unknown cause, in patients with other principal diagnoses: the first, a 25-year-old patient suffering from systemic lupus erythematosus; and the second, a 31-year-old woman diagnosed with rectal adenocarcinoma.^2,3 Both progressed favourably with prescribed empirical treatment.

M. atlantae is a short, immobile, Gram-negative bacillus that grows in standard culture media. It is catalase- and oxidase-positive, cannot acidify sugar, is negative for nitrate reduction and assimilates acetate and nitrate. It is alkaline phosphatase-positive and pyrrolidone carboxyl peptidase-positive.² It is underdiagnosed because it is difficult to isolate using classic methods and due to its sensitivity to routine antibiotics.

Despite continuous advances in microorganism identification using classic phenotyping techniques, such as the API^® HK method (BioMérieux, Marci L’ Étoile, France), or by automated methods, the identification of certain species like M. atlantae continues to pose a challenge both due to difficulties with the culture as well as the excessively long identification time.⁴ For these reasons, and to prevent diagnostic delays, new rapid-detection methods are being developed, such as MALDI-TOF mass spectrometry.⁵

The first reference to this method of bacterial identification dates back to 1996 and was performed by Holland et al.⁶ and Krishnamurthy et al.⁷. As well as being an easy and cost-effective method, it is also the quickest method to identify bacteria in blood cultures. Its efficacy varies from 43% to 94% depending on the pathogen.

By analysing proteins, particularly ribosomal proteins, this technology can identify microorganisms from colonies or directly from samples by creating a mass spectrum (that is specific to each species). It can identify both the genus and species of microorganism depending on the reliability score, the limits of which are set by the manufacturer (<1.7 not reliable for genus or species, 1.7–2 reliable for genus, not for species, >2 very reliable for both genus and species).⁸

Its high cost and the need to produce an antibiogram using classic methods constitute its main limitations.

To address these limitations, new antibacterial resistance rapid-detection techniques, such as detection of beta-lactam antibiotics and methicillin-resistant Staphylococcus aureus strains by MALDI-TOF, are being developed.⁹

Funding

The authors did not receive funding of any kind for this study.

Conflicts of interest

The authors declare that they have no conflicts of interest.