Impact of aerosol dwell time on the survival of Mycobacterium tuberculosis

The airborne pathogen Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis (TB) and claims about 1.6 million deaths annually (WHO, 2022). TB is transmitted by inhalation when a person with active pulmonary TB produces aerosols, by coughing or other respiratory maneuvers, containing viable Mtb, and a contact inhales these particles. Aerosolized Mtb released by the source case must first survive in air before it can be inhaled by a contact person. The length of time that Mtb can stay viable in air is therefore important for its transmission. While airborne, Mtb is faced with dehydration stress, which is affected mainly by ambient humidity and temperature. However, the longevity of Mtb in aerosols under set environmental conditions has not yet been investigated. Here, the H37Rv laboratory strain of Mtb was aerosolized with a Collison-type nebulizer (CH Technologies, USA) inside a tailored, airtight chamber. The chamber was set at defined conditions of relative humidity (RH) and temperature. At 0, 8 and 30 minutes after aerosolization, Mtb aerosols in the chamber were collected for 10 min using a 6-stage Andersen cascade impactor (TISCH Environmental, USA). Colony-forming units (CFUs) of Mtb from aerosol samples were determined by culture on mycobacterial-selective 7H11 agar 3 weeks later. Culturability of Mtb aerosols at 40% RH and temperatures ranging from 22 to 26 °C (low absolute humidity [AH]) was variable, with 57% of aerosol collections resulting in positive regrowth on agar. In contrast, culturability of aerosolized Mtb was 100% when experiments were performed at 60% RH, 22 °C or 60% RH, 26 °C. Recovery of Mtb H37Rv at low AH decreased over time, with a trend of approximately 1 log reduction in CFUs from 0 to 30 min. At middle and high AH, reduction in CFUs between 0 and 30 min of aging was only 0.5 and 0.61 log, respectively. The results shown in this work are first-of-a-kind experiments with aerosolized Mtb and they suggest that its survivability and longevity in aerosols improves in higher humidity settings. This work aims to shine light on the knowledge gaps of Mtb transmission by studying the conditions under which the bacteria survive the best and longest in the air, increasing its chances of finding a new host. Our work suggests that higher humidity, which is common in many TB endemic countries, favor Mtb survival in aerosols. Conditions of higher indoor humidity, which reach 15 g/m3 AH in endemic countries, may thus present an increased risk for TB transmission when sharing air with persons having active pulmonary TB. Understanding pathogens, their mode of transmission and the determinants of epidemiological success plays a central role in developing effective tools for containment and the long-term goal of disease eradication.