Description of mycobacteria
A Review of Mycobacteria to Research Optimal Growth Conditions of Clinical and Environmental Mycobacteria
There are few organisms that are as old and important as bacteria. As technology has improved, more and more different species of bacteria have been identified. In addition, there have been bacteria that have been identified as beneficial, harmful, or that do not seem to affect humans or animals. Mycobacterium, a genus of bacteria, is commonly associated with the microorganisms that cause tuberculosis and leprosy, but also contains many that exist in the environment freely. These nontuberculou environmental mycobacteria will be the focus of the honors project. The experimental design research will evaluate water quality and other conditions that may be related to the growth rate of mycobacteria of clinical significance.
Description of Mycobacteria
The genus Mycobacterium contains a diverse collection of both clinical and environmental mycobacteria. They are characterized as aerobic, slow-growing, and exist in every natural ecosystem. Clinical mycobacteria are identified as members ofMycobacterium leprae and Mycobacterium tuberculosis complex. They do not exist freely in the environment, only within other organisms. Environmental mycobacteria, sometimes described as atypical, contain species that are both pathogenic and nonpathogenic to humans and animals. Infections of this kind are considered to be environmentally-derived (Iivanainen, Martikainen, Vaananen, & Katila, 1993). Currently, there are 91 species of environmental mycobacteria identified. Only about 30% of mycobacteria isolated from the natural environment are from known species. This suggests that there are many new species left to be discovered (Primm, Lucero, & Falkinham, 2004).
Mycobacteria and their Cell Wall
Mycobacteria are considered unique because of their special cell wall. It is very thick and waxy, as opposed to many other kinds of common bacteria. Due to this, it has a very low permeability and unusual structure (Jarlier & Nikaido, 2006). Its pores are not well suited to the transport of materials and solutes across its membrane. In addition, lipophilic agents are slowed down in its low-fluidity lipid bilayer. This inefficient transfer may be related to its slow growth.
Growth of Mycobacteria
Mycobacteria have a very low growth rate. Limited numbers of ribosomes for protein synthesis, high energy requirements, and low permeability of the cell wall contribute to the slow growth (Falkinham, Nichols, Bartram, Dufour, & Portaels, 2004). There are both slow-growing and fast-growing types of environmental mycobacteria. Although, the fast-growing kinds are still much slower than other bacteria. Development of a colony in less than seven days is considered to be fast, whereas, development over seven days is slow (Primm, et al., 2004). Despite the cell walls contribution to slow growth, the presence of a hydrophobic barrier facilitates the growth of mycobacteria in environments where they would normally be washed out mechanically. This allows them to persist in locations where other bacteria may not be able to remain. The slow growth of mycobacteria is also beneficial to their persistence (J.O. Falkinham, 2009).
Mycobacteria and Resistance
The combination of mycobacteria's unique cell wall and its slow growth greatly contribute to disinfectants and antibiotics. Atypical mycobacteria are oligotrophs and are able to grow many different kinds of organic compounds that can interfere with the mechanisms of these medications. (J.O. Falkinham, 2009). Since the cell wall does not allow the transference of particles across it very easily, this barrier prevents many agents from reaching the cell and causing death. Mycobacteria seem to be resistant to heavy metals and oxyanions. As a result of slow growth, the cells are allowed to trigger mechanisms that prevent the disinfectants from causing permanent damage (Falkinham, et al., 2004). Varying levels of resistance to disinfectants are exhibited by different kinds of mycobacteria. For example, some species have lesser resistance to chlorine than others (Primm, et al., 2004). The most susceptible species of mycobacteria to chlorine are M.aurum and M. gordonae. Interestingly, these species are still 100 to 330 times more resistant to chlorine than E. coli (LeDantec et al., 2002). Mycobacteria's extraordinary resistance to disinfectants and antibiotics plays a large role in the environment it thrives in.
Where Mycobacteria Are Found
Mycobacteria exist in and around surfaces in soils, fresh and saltwater, and in both human and animal hosts. This widespread distribution constitutes a need to further understand their ecology and growth conditions in order to relate this to the growing number of environmental mycobacteria infections (O'Brien et al. 1987; Primm et al., 2004).
Nontuberculous Environmental Bacteria
Mycobacteria species other than Mycobacterium leprae and Mycobacterium tuberculosis complex are free-living in the environment (Vaerewijck, Huys, Palomino, Swings, & Portaels, 2005). Of the environmental mycobacteria found in water, there are certain conditions that are yet to be explored in the laboratory that account for differences in concentration of the bacteria.
Parameters in Water Quality For Mycobacteria
It is generally assumed that certain water quality conditions will enhance mycobacteria growth: dissolved organic carbon, temperature, pH, phosphorus, iron, zinc, and turbidity (Carson, Peterson, Favero, & Aguero, 1978; Falkinham, et al., 2004; Iivanainen, et al., 1993; Kirschner, Parker, & Falkinham, 1999; Primm, et al., 2004; Torvinen, Lehtola, Martikainen, & Miettinen, 2007; Vaerewijck, et al., 2005). It has been shown that ideal pH levels are between four and six, denoting a preference for acidic waters(Carson, et al., 1978). In terms of water temperature, there is a wider range of tolerable conditions. For example, mycobacteria have been found in places including hot tap water and ice machines (Katila et al, 1995). A temperature of 20°C is required for mycobacteria to be able to reproduce (Kirschner, et al., 1999). In environments such as soils rich in peat and dissolved organic carbons of fulvic and humic acids were shown to foster mycobacterial growth (Iivanainen, et al., 1993; Kirschner, et al., 1999). Low oxygen environments, such as soil and water, return copious samples of mycobacteria. Concentrations between four and 21% of atmospheric levels frequently show existence of mycobacteria, although there are some species can live in anaerobic conditions (Falkinham, et al., 2004; Vaerewijck, et al., 2005). Decreased numbers of recovered mycobacteria have been associated with increased levels of phosphorus. Hetereotropic bacteria thrive in these conditions, therefore, contributing to competition with mycobacteria, which reduce their numbers (Torvinen, et al., 2007). Increased numbers of recovered mycobacteria have been noticed to correlate with higher levels of zinc. However, this relationship is poorly understood (Falkinham, et al., 2004; Vaerewijck, et al., 2005). Tap water used for drinking contains higher amounts of iron and is related to higher amounts of recovered mycobacteria. Since iron does react with free chlorine in drinking water, this may subtract from the seemingly causal relationship with growth of mycobacteria. This interaction requires further study (Vaerewijck, et al., 2005). Finally, a higher level of turbidity has been found to foster mycobacterial growth because there are more suspended particles which create more surfaces in which mycobacteria may create biofilms (Primm, et al., 2004).
Creation of Biofilms
The cell wall of mycobacteria is the most hydrophobic of all bacteria. As a result, mycobacteria tend to conglomerate at locations where there is a junction between water and another surface, for example, water and stone. The mycobacteria replicate outward at these junctions creating a biofilm. Biofilms such as these exist almost everywhere where water and a different surface interact (Falkinham, et al., 2004).
Drinking Water Systems
Mycobacteria exist in drinking water systems everywhere in the world (Torvinen, et al., 2007). There are many factors that contribute to their ability to thrive in these conditions: resistance to heat and disinfectants, creation of biofilms, and an affinity for high concentrations of zinc (Falkinham, 1996; Vaerewijck, et al., 2005). Their resistance to elevated temperatures allows them to thrive in conditions that other bacteria may not be able, thereby reducing competition. Chlorination is the example of a conventional method used to disinfect drinking water systems. However, mycobacteria are largely unaffected by this technique. When biofilms are created in the surfaces related to drinking water systems, mycobacteria are able to proliferate to other areas. Finally, most drinking water system pipes are galvanized, creating a surface covered with a zinc conserving to an increase in mycobacteria growth (Torvinen, et al., 2007).
Infections in Humans and Animals
Tuberculosis and leprosy are the two most well-known diseases associated with mycobacteria. Many other diseases in humans and animals, though, are the result of environmental mycobacterial infections.
Human Mycobacterial Infections
Environmental mycobacteria exist in nearly every medium and ecosystem in the world. One a host comes in contact with this bacteria, it is able to enter its body through a number of means: gills, mouth, nose, open wounds, and any other orifice. Initially, the immune system of the host signals macrophages to digest the incoming mycobacteria (Cosma, Sherman, & Ramakrishnan, 2003). The invading bacteria is able to survive due to its cell wall preventing it from being digested by the macrophages lysosomes. This in turn allows the mycobacteria to use the surrounding tissue of the macrophage as a vessel for spreading through the body of the host. This means of spreading has been studied through the use of mice and rabbits embryos. When the mycobacteria spread, many different kinds of infections may result (Dannenberg, 1993).
Known Mycobacterial Infections in Humans
Mycobacterium lepraeis the cause of leprosy. This is species of mycobacteria has been recorded in many civilizations throughout history including the ancient Chinese and Egyptians (Barnhart, 1995). Leprosy is the disease of the upper respiratory tract including the mucosa and peripheral nerves (Ryan & Ray, 2004). A multi-drug approach is used when treating leprosy because it has an uncanny ability to become resistant to a single treatment (Ishii, 2003).
Mycobacterium tuberculosis complexis a widespread species of mycobacteria that causes the disease tuberculosis. Currently, about one third of the world's population is infected with tuberculosis (Thomas, 2005). The vast majority of tuberculosis cases are pulmonary in nature, whereas, a smaller amount of cases purvey when the infection spreads to other parts of the host's body, called extra pulmonary tuberculosis. Chest pain and prolonged cough are hallmark symptoms of tuberculosis (Golden & Vikram, 2005). In humans, inhalation into the lungs is the most common form of contraction. In response to this, the immune system creates granulomas, which are organized immune structures. Necrosis of the tissue results and death of the host can ensue if necrosis interferes with functionality (Cosma, et al., 2003). Environmental mycobacteria are also a major cause of disease.
Mycobacterium ulcerans is the leading mycobacterial cause of infection in the world next to Mycobacterium tuberculosis complexand Mycobacterium leprae(Kaattari, Rhodes, Kaattari, & Shotts, 2006). Toxins are produced that that result in necrotic ulcers were the bacteria grows (Cosma, et al., 2003). Called Buruli ulcers, if left untreated, it can lead to inflammation of the limbs, pain, disfiguration, fever, and even death (Werf, Stinear, Stienstra, & Graaf, 2003).
Mycobacterium avium complexinclude many species of nontubercular mycobacteria. It has been suggested that this could be a possible cause of Crohn's disease (Harris & Lammerding, 2001). Inflammation of the host's gastrointestinal tract ensues and leads to abdominal pain and diarrhea (Staff, 2008). The majority of patients with Crohn's disease respond well to anti-mycobacterial therapy (Primm, et al., 2004).
Autoimmune disorders such as allergic reactions are thought to result from the lack of mycobacteria (Black, 2001). Mycobacterial vaccinations such as, Bacillus Calmette-Guérin, has been shown in mice to reduce airway allergic responses by signaling a TH1 response. This vaccinations there be is being proposed as a possible treatment for human allergies (Choi & Koh, 2002).
Human immunodeficiency virus (HIV) infections in humans causes them to be even more susceptible to mycobacterial infections. They may act as opportunistic pathogens in immune-compromised individuals whereas they would not affect healthy individuals (Falkinham, 1996). As a result, 25% to 50% of individuals with AIDS also have environmental mycobacteria infections (Horsburgh, 1991). For most individuals without HIV, pulmonary disease is the most common result of mycobacteria infection. Treatment for these infections are difficult because they spread throughout the body (Falkinham, 1996).