Clostridium perfringens: The Agricultural Significance and Hazards Posed when Virulence and Resistance Combine
Clostridium perfringens is among the most ubiquitous pathogens known. C. perfringens is considerably hardy and found in soil, food, water, fecal material, and the intestines of both humans and animals (Uzal et al. 2014). This widespread bacterium is characterized by being a spore-forming anaerobe, which is both Gram positive, and rod-shaped (Kiu and Hall 2018). The importance of this pathogen lies within both its ability to produce a variety of deadly toxins, and the growing concern of antibiotic resistance throughout the globe. C. perfringens is most commonly mentioned for its causation of toxin-dependent infections, such as clostridial myonecrosis, otherwise known as traumatic gas gangrene (Li et al. 2016). However, the complications and breadth of damage that this bacterium causes are not confined to a single method of infection. This highly virulent pathogen has roughly 20 degradative toxins at its disposal, some of which cause more harm than others (Kiu et al. 2017). C. perfringens can infect a multitude of hosts, and each case will vary dependent on the organism, route of infection, and the toxins which are released. While the issues of direct contact with this bacterium are severe, the agricultural impact that is posed by this pathogen threatens nearly every level of food production. From farm fields, to storage, and especially food handling and distribution, C. perfringens cells and toxins are very real threats to quality and safety control aspects of global markets (Hall and Angelotti 1965). These hazards are further exacerbated by the potential for antibiotic resistance to prevent proper treatment of infections. The overuse of antibiotic growth promoters in animal feed for cattle and poultry, among other animals, has led to their antibiotic properties to become ineffective against a variety of pathogens, such as C. perfringens (Gaucher et al. 2017). The ability for bacteria to engage in horizontal gene transfer (HGT) has been extensively studied yet seemingly overlooked and understated in the agricultural business. Keeping farm animals healthy and free of pathogens are among the primary reasons for ample use of AGPs. However, this comes at the risk of stronger, more complex, versions of bacteria to arise and endanger food production (Lacey et al. 2017). Like many bacterial pathogens, C. perfringens ability to alter its outer protein structure when overexposed to certain antibiotics allows for dangerous adaptations in which new antibiotics must be synthesized; which is very costly and not in the interest of many pharmaceutical companies (Uzal et al. 2014). C. perfringens, along with its many toxins and growing resistance to available treatments, is a threat to modern agriculture which must be closely monitored and researched in efforts to prevent outbreaks of resistant strains. It is the responsibility of governments, food producers, and regulatory organizations to understand the potential risks of overusing antibiotics. The agricultural battle to prevent contamination and the spread of C. perfringens begins with properly understanding the arsenal of toxins within both its genome and conjugative plasmids.
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C. perfringens toxins are separated into different groups based on the diseases they are associated with, the extent of damage they cause, and which toxin or combinations of toxins are normally employed; the groups are identified as A-E. These categories are used when identifying the four major types of bacterial toxins: alpha, beta, epsilon, and iota (Grenda et al. 2017). In addition to these, there are two other clinically significant types named beta-2 and enterotoxin. These major types are accompanied by 12 minor toxins, which are not attributed to play a major role in disease, but nevertheless are harmful to both humans and animals (Fohler et al. 2017). Each toxin is uniquely identified by the gene it carries, allowing for proper genetic and molecular research into the inner workings of toxin production and pathogenicity. Arguably, the most agriculturally prevalent is alpha toxin. Alpha toxin is the major cause of symptoms and damage associated with histotoxic infections such as gas gangrene (Fohler et al. 2017). Furthermore, the gene which encodes to produce alpha toxin, cpa, is highly-conserved and found in all strains of C. perfringens (Li et al. 2016). In circumstances where alpha toxin is combined with a subtype of toxin, named NetB, avian necrotic enteritis in poultry can occur, which is quickly spread and is both a safety and economic burden (Li et al. 2016). Lesser attributed, yet still important, is the ability for alpha toxin to cause human food poisoning along other gastrointestinal diseases. Beta, epsilon, and iota toxins all have significant roles in agriculture as well.
Encoded by the cpb gene, beta toxin is attributed to the cause of necrotic enteritis in a variety of species. In instances where this toxin is released by C. perfringens near cattle, lamb, and similar species, enterotoxaemia and dysentery can plague whole farms (Fohler et al. 2017). It is vital for the survival of many farm animals that beta toxin be prevented from contaminating animal food; and the spreading of the C. perfringens infections must be identified and controlled in a timely manner. Epsilon toxin, which is encoded by the gene etx, affects sheep and goats. This toxin is best known for its contribution in the cause of enterotoxaemia, more so in the beforementioned animals, but can occur in cattle (Li et al. 2016). Epsilon toxin belongs to groups D and B of the major toxin categories and is not as commonly found in field isolates as the other types of C. perfringens toxins. The last major toxin, iota, is encoded by the iap and ibp genes, and is found only in group E. Iota toxin has an agricultural impact on cattle, sheep, and rabbit (Li et al. 2016). Although not entirely known, it is suspected that this toxin is also a major cause of enterotoxaemia in many farm animals. Along with their many similarities, these toxins are unique in their combinations of a variety of toxin-encoding genes, virulence, and prevalence in the environment. The consequences of animal feed and livestock contamination are the driving force to eliminate C. perfringens vegetative cells and spores from destroying the hard work of livestock producers. In addition to these major toxins, C. perfringens has very significant beta-2 toxin and enterotoxin at its disposal. Beta-2 toxin is recognized for inducing necrotic enteritis in young pigs, human associated non-foodborne diarrheal illnesses, and multiple gut diseases (Kiu and Hall 2018). This potent toxin has various impacts in agriculture and has developing research into its role in human infections. The most important toxin, enterotoxin, is encoded by the cpe gene. Enterotoxin is produced by C. perfringens and is responsible for a variety of human gastrointestinal diseases (Kaneko et al. 2011). Fortunately, enterotoxin is the least prevalent of the toxins produced by C. perfringens. In many outbreaks, enterotoxin is responsible for only 5% of the discovered bacterium population (Kaneko et al. 2011). The significance of this toxin cannot be understated. Enterotoxin is capable of binding to human cells and inducing apoptosis (Kiu and Hall 2018). However, the toxins mentioned here are not only an issue in cattle, poultry, and other animal farms; they pose threats to vegetable producers, storage and transportation, and the handling and distribution portions of agriculture.
While C. perfringens contamination has the potential to begin at an animal breeding facility, it has just as much of an impact and opportunity to do so in the environment. Both vegetative cells and spores can be found on fresh produce, in fecal material, and in various food particles that may not be hospitable hosts; yet these may serve as transmission vectors onto machinery and handler gloves (Bryan and Kilpatrick 1971). Once contamination of a machine, blade, tool, or handler has occurred, the rapid spread of toxin containing cells can be dispersed into such large amounts of food that a catastrophic amount of people can become infected before isolation measures are taken. Ultimately, the introduction of C. perfringens into already cooked foods is of greatest importance (Talukdar et al. 2017). There are a few heat-resistant strains of this pathogen which pose heightened concern among food distributors. Proper cooking may get rid of most C. perfringens cells, however strains which are resistant to normal temperatures of (60° C) may become heat-shocked and induced into rapid germination (Hall and Angelotti 1965). For these reasons, precautions along all areas of food production, from growth and breeding on farms or facilities, to finalizing a customer’s dish, proper sanitation and safety regulations must be followed. Once C. perfringens has been detected, there are methods to determine which strains and toxins are being produced; this is vital to selectively treat the situation. Measures to identify which toxins are infecting animals include the use of the polymerase chain reaction (PCR) and pulsed-field gel electrophoresis (Lacey et al. 2017).
Different methods of PCR, such as nested, real-time, and loop-mediated isothermal amplification (LAMP) PCR, exist to quantify results under various temperatures and bacterial concentration conditions. The goal of these molecular assays is to detect and amplify bacterial DNA for reliable identification of the cause of a pathogenic surge (Kaneko et al. 2011). It is often difficult to determine the source of a C. perfringens outbreak. The methods used are categorized as molecular source tracking, which seek to determine where a toxin-producing strain of a pathogen originated; because C. perfringens is so widespread within the environment, proper determination of a single source of infection requires detailed information on the location where the first patients were infected (Kaneko et al. 2011). Treatment for an outbreak cannot begin until the pathogen responsible is correctly identified. Based on C. perfringens virulent strains and toxins, there is no time to be wasted when a threat has occurred; whether it be on a farm, or in the population, direct action must be taken immediately. Molecular assays, enrichment steps, and gel electrophoresis are useful for the amplification and identification of certain C. perfringens positive foods and isolates when low numbers of bacteria are found (Kaneko et al. 2011). These methods are useful during foodborne outbreaks in which a population has been infected and the pathogen has been linked to a source. Enrichment or induced multiplication of the pathogen is often needed before performing gel electrophoresis, to prevent the occurrence of false-negative results (Lacey et al. 2017). While identification and treatment during outbreaks is very necessary, preventing infection in the first place causes much less of an economic and logistical burden.
Prevention of pathogenic diseases is a very organized and delicate field of regulations, restrictions, and constant observation. C. perfringens and other virulent pathogens must be handled and monitored with extreme care. Certain prevention measures include vaccines, therapeutics, and antibiotic treatments (Kiu and Hall 2018). However, for many pathogens such as C. perfringens, there isn’t necessarily a vaccine that can be taken, and antibiotic treatment has most recently been abused and qualitatively caused more harm than good. Organizations such as the World Health Organization (WHO) respond to and keep a detailed record of previous foodborne outbreaks. It has been estimated that in 2010 there were roughly 3,998,164 foodborne illnesses caused by C. perfringens (Kirk et al. 2015). Preventing the contraction of this pathogen can be as simple as requiring food handlers to properly sterilize equipment after each use, or to wear proper gloves and hairnets. Food contamination begins with improper safety techniques and overlooked risks, with the benefit of speed and mass production, but at the cost of potential outbreaks. However, a variety of chemical agents such as nitrate can be used as preservatives to combat C. perfringens (Talukdar et al. 2017). Research and development are vital in the race against microbial adaptations; finding new methods of inactivating pathogenic spores and preventing vegetative cell growth are just a couple areas of expertise that are constantly evolving. Nitrates, along with organic acids, phosphates, natural antimicrobials and even essential oils have been shown to offer partial resistance to C. perfringens (Talukdar et al. 2017). The most common, yet controversial means of combating microbial pathogens has been the worldwide overuse of antibiotics.
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Antimicrobial resistance (AMR) is an emerging threat for various livestock animals and humans. Treatment of many pathogens, including C. perfringens, is becoming increasingly difficult with the emergence of new antimicrobial resistant pathogens (Kiu and Hall 2018). There is a finite amount of antibiotics at the disposal of pharmaceutical and medical professionals. When a pathogen becomes adapted to an antibiotic, it is no longer useful for treatment. Despite this, in the early 2000’s, antibiotic growth promoters were introduced into farm animal feed stock at a staggering amount of 24.6 million pounds for non-therapeutic purposes to prevent contamination (Fair and Tor 2014). This is a pressing issue and is undoubtedly affecting agricultural business both economically and ethically. The preservation and growth of livestock requires a heightened level of control, which one could argue that the use of antibiotics is necessary for preventing mass infection in farm animals; however, this only works if pharmaceutical companies are readily synthesizing new antibiotics which pathogens will have no prior immunity to (Fair and Tor 2014). It is simply not in pharmaceutical companies’ self-interest to produce antibiotics. They are costly to create and are only prescribed for a duration of a couple of weeks. When compared to long-term drugs that treat chronic illnesses, antibiotics are not worth the investment required to pursue (Fair and Tor 2014). AMR has been confirmed in a variety of C. perfringens species which have adapted against tetracycline, gentamycin, and more notably detected the occurrence of mepA, a gene encoding for multi-drug resistance; it is evident that whole genome sequencing techniques will play a vital role in the preparation and development of new antibiotics to treat resistant strains of C. perfringens (Kiu and Hall 2018). Monitoring the everchanging protein structures on pathogen surfaces and staying updated on bacterial genomes are key factors to preventing a future outbreak. The extensive use of antibiotics to keep livestock healthy is not the only cause of growing AMR, doctors have been contributing to its rise for decades.
The over-prescription of antibiotics to treat patient symptoms has played a hefty role in resistant strains of pathogens. The Center for Disease Control (CDC) estimates that 50% of all antibiotics are prescribed unnecessarily, and at a yearly cost of roughly $1.1 billion in the United States, alone (Fair and Tor 2014). Based on these results, changes are occurring in the legal system to prevent catastrophe from AMR pathogens. New programs are arising to address issues of antibiotic resistance. Furthermore, plasmid-mediated conjugative transfer of AMR genes has and will continue to produce more virulent strains of C. perfringens (Gaucher et al. 2017). It is important to note that bacteria have been evolving against natural antibiotics far before any human interference. Bacterial DNA has been isolated from over 30,000 years ago and have been proven to be resistant to natural antibiotic products, and when compared to current strains, bacteria have had much more time to grow in their resistance, than humans have had to synthesize new antibiotics (Fair and Tor 2004). The nature of bacteria, their rapid growth and adaptation, has led to interesting, yet dangerous strains of pathogens to develop. It is a combination of these traits, and the human misuse of antibiotics, which have placed unintentional pressures on bacteria to produce AMR strains, and to ultimately thrive in the absence of antibiotics (Fair and Tor 2004). The most pressing danger lies in C. perfringens ability to transform nonpathogenic strains into virulent, and potentially fatal, necrotic enteritis causing strains. Resistance related genes, virulence factors, multiple enzymes and toxins, have the capability to be included in the accessory genome of C. perfringens (Lacey et al. 2017). This allows for the risk of AMR strain development to arise in the presence of the human misuse of antibiotics; whether by farmers, medical professionals, natural adaptations, or a failure of collective responsibility, C. perfringens will eventually outcompete modern antibiotics.
Food production relies on multiple levels of protection from harmful pathogens, contamination, and quality control. Demand for rises in the production of beef and chicken, along many other foods, has caused an increase in the widespread use of antibiotics (Uzal et al. 2014). Antibiotic resistance is not anything new to this era, and in fact affects many pathogens unrelated to C. perfringens. The attributes which constitute C. perfringens to be an agriculturally significant pathogen include its ubiquitous living conditions, its diverse arsenal of lethal toxins, and its history of economic burden for farmers, medical professionals, and patients alike (Talukdar et al. 2017). The ability for C. perfringens to occupy a variety of living conditions complicates the treatment of foodborne outbreaks. Narrowing the source of an infectious pathogen relies on extensive laboratory testing; this is a challenge with hardy, widespread pathogens. The multiple toxins which C. perfringens has at is disposal further complicates treatment of both animal and human outbreaks. Determining which strain, or if multiple strains, of C. perfringens are involved in a single outbreak is a challenge. However, by identifying the toxins which are released, it is possible to narrow the search and apply known antibiotics or other chemical agents for treatment. The process of inactivating toxins can take years of testing, resources, and capital to pursue (Fair and Tor 2014). Pharmaceutical companies, and researchers alike, have a responsibility to pursue the creation of new antibiotics. Although costly, it is necessary to form new treatments against a variety of pathogens. C. perfringens is just one example that will forever be constantly adapting and forming resistant strains. The agricultural battle to prevent foodborne outbreaks begins with bringing attention to the consequences that will result from a lack of action. Becoming well-educated and raising awareness towards these issues are productive steps, which must be taken, to outcompete C. perfringens ability to contaminate global markets.
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