RESPIRATORY
IN RECENT YEARS, THERE HAS BEEN A TEN-FOLD INCREASE IN INFECTIONS CAUSED BY NONTUBERCULOUS MYCOBACTERIA (NTM). THIS IS A GROWING CONCERN IN THE UK AS LITTLE IS UNDERSTOOD ABOUT THE ORGANISMS AND, WITH THE CHALLENGES OF ANTIBIOTIC RESISTANCE, DIAGNOSIS AND TREATMENT ARE PROVING TO BE EXCEPTIONALLY DIFFICULT.
NONTUBERCULOUS MYCOBACTERIA: A GROWING CONCERN FOR THE UK
By Chanel Jones N
ontuberculous mycobacteria (NTM), also known as atypical mycobacteria
or environmental mycobacteria, refer to a group over 150 species1 of less pathogenic mycobacteria that do not cause tuberculosis (Mycobacterium tuberculosis) or leprosy (Mycobacterium leprae). These bacteria are not normally infectious; however, we are now seeing an increase in infectious cases of this group of bacteria in patients with underlying respiratory diseases such as chronic obstructive pulmonary disease (COPD) and cystic fi brosis2
,3 and in
patients with weakened immune systems1.
MICROBIOLOGY Mycobacteria are aerobic, rod-shaped bacilli that are found mainly in soil and water sources1
. The mycolic acids, for
which the mycobacteria are named, make up more than 60 per cent of the
total cell wall mass and are distinctive for each species4
(see image 1).
Because of their cellular structure they are classifi ed as gram-positive bacteria; however, they do not stain due to the lipid rich, hydrophobic cell wall, which is impermeable to hydrophilic stains, disinfectants and antibiotics1
. As a
result, mycobacteria are often referred as acid-fast bacteria, as they can only be stained by prolonged application of concentrated dyes, facilitated by heat. As they cannot be decolourised, they strongly retain the stain4
.
Though there are more than 150 different species of mycobacteria, pulmonary infections are mostly due to the presence of Mycobacterium avium-intracellulare complex (MAC), Mycobacterium kansasii, and Mycobacterium abscessus2
. It
is important to know the bacteria involved and its growth rate (see image 2) as it aids in treatment selection.
Image 1. Sourced from:
www.nature.com/nrmicro/journal/v13/n10/full/ nrmicro3480.html5
EPIDEMIOLOGY
The incidence of NTM in England, Wales and Northern Ireland more than doubled between 1996 and 20063
is continuing to rise; between 2007 and 2012, 21,118 individuals tested positive for NTM culture isolates. However, this trend is not seen in Scotland, where there has been no change recorded from 2000 to 20103
For the majority of people, NTM is cleared naturally from the airway and does not cause infection; however certain patients are more at risk (see image 3). NTM is very rarely transmitted from person-to- person. Evidence shows that the infection is acquired mainly from the environment, and it is hypothesised that the pulmonary disease is caused by inhalation of aerosols of water containing the bacteria1
.
SIGNS AND SYMPTOMS Signs, symptoms and severity can vary from person to person. The most common manifestation of NTM is NTM pulmonary disease (NTM-PD). The symptoms are similar to those seen in other lung and respiratory illnesses, which include: cough, shortness of breath, coughing up blood, excess sputum production, fever, weight loss,
and wheezing and chest pain. .
The two main clinical presentations of NTM-PD are: nodular bronchiectasis and cavitary disease. Nodular bronchiectasis, or diffuse bronchiectasis, is irreversible and abnormal dilatation of the airways caused by infl ammation, and it usually affects the predominantly the middle and lingular lobes of the lungs7
. Infl ammation is triggered
when neutrophils in the lumen of the bronchi release infl ammatory mediators in response to the NTM8
.
The infl ammatory mediators assist in the destruction of elastin, cartilage and muscle of bronchi resulting in irreversible dilation. At the same time, lymphocyte and macrophage infi ltrates cause thickening of the mucosal wall in the small and medium-sized airways8
. In addition, there is excess
mucus production, cilia damage and a reduction in ciliary motility, trapping the NTM bacteria in the lung, leading to chronic infection.
Nodular bronchiectasis is mainly seen in nonsmoking, middle-aged or elderly (postmenopausal) caucasian women with low BMIs. These women have no previous history of lung disease; however, they suffer from a
Image 2. Sourced from:
http://lalashan.mcmaster.ca/theobio/projects/ images/c/c0/An_Introduction_to_Infectious_Diseases.pdf4
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18 - SCOTTISH PHARMACIST
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