Mycobacterium Terrae: Infections, Diagnosis, and Treatment

Mycobacterium terrae is a species of nontuberculous mycobacteria (NTM) found throughout the environment. M. terrae is widespread in nature, but it is considered an uncommon cause of human illness. Its name, derived from Latin, means “of the earth,” reflecting its common presence in soil. Initially, it was considered nonpathogenic after early studies in animals showed no signs of illness; however, later findings confirmed its ability to cause infections in humans.

Natural Habitat and Transmission

This bacterium is ubiquitous in the environment, thriving in soil, dust, and various water sources. It has been isolated from natural bodies of water and municipal tap water systems, where it can persist and become a source of exposure.

Human exposure occurs through direct contact with these contaminated environmental sources, as the bacterium is not transmitted from person to person. A primary route of infection is through direct inoculation, where the bacteria enter the body through a break in the skin caused by trauma or injury. Surgical procedures that use contaminated instruments or solutions can also introduce the bacterium into the body. Another mode of transmission is the inhalation of aerosolized particles from contaminated water or soil.

Activities like gardening or other work involving soil can increase the risk of skin inoculation. Similarly, the use of humidifiers that create aerosols from contaminated tap water can be a source of airborne exposure. Although contact is common, the development of an actual infection is infrequent and linked to specific vulnerabilities.

Associated Human Infections

Infections caused by Mycobacterium terrae are opportunistic, primarily affecting individuals with compromised immune systems or those who have sustained significant physical trauma. The most common disease associated with this bacterium is tenosynovitis, which is an inflammation of the sheath surrounding a tendon. These infections frequently occur in the hand and wrist and often develop after a penetrating injury that introduces the bacteria into the deeper tissues.

Beyond tenosynovitis, M. terrae can cause a range of other conditions. Skin and soft tissue infections, such as abscesses, may develop, particularly at the site of an injury. In some cases, the infection can penetrate deeper, leading to osteomyelitis, an infection of the bone. Although less common, pulmonary disease from inhaling the bacteria has also been documented. These lung infections can present with symptoms like a persistent cough.

The presentation of these infections is often slow and gradual, which can make diagnosis challenging. Disseminated disease, where the infection spreads throughout the body, is rare but has been reported in severely immunocompromised individuals. The majority of cases remain localized to the site of initial exposure.

Diagnosis and Identification

Diagnosing a Mycobacterium terrae infection begins with collecting a clinical sample from the affected area. For skin or tendon infections, this typically involves a tissue biopsy from the infected site. In cases of suspected pulmonary disease, a sputum sample or fluid obtained through a bronchial wash is collected.

The identification process in the laboratory can be lengthy because mycobacteria are slow-growing organisms. Cultures of the submitted samples may need several weeks of incubation before bacterial growth is visible. Once growth occurs, laboratory specialists use specific techniques to distinguish M. terrae from other species of nontuberculous mycobacteria and, importantly, from Mycobacterium tuberculosis, the bacterium that causes tuberculosis.

Traditional identification methods have relied on observing the colony’s morphology and performing a series of biochemical tests. However, modern molecular techniques are now more commonly used for precise identification. Methods such as polymerase chain reaction (PCR) or gene sequencing can provide a more rapid and accurate identification of the bacterial species, allowing for targeted medical treatment.

Treatment Strategies

Treating infections caused by Mycobacterium terrae is often a complex and prolonged process. This bacterium demonstrates natural resistance to many of the first-line antibiotics that are used for treating tuberculosis. The choice of antibiotics is often guided by susceptibility testing performed on the specific bacterial isolate from the patient.

A common treatment regimen involves a multi-drug approach to prevent the development of further resistance and effectively clear the infection. Medications frequently used include clarithromycin, rifamycins such as rifabutin, and ethambutol. Patients must continue this combination therapy for an extended period, often 6 to 12 months or longer, to fully eradicate the infection and prevent relapse.

In addition to antibiotic therapy, surgical intervention may be necessary in certain cases. For infections involving abscesses or significant tissue damage, such as severe tenosynovitis, surgical debridement is often performed. This procedure involves the removal of infected and necrotic tissue, which helps to reduce the bacterial load and allows antibiotics to penetrate the remaining tissues more effectively.