Compared to eukaryotic cells, bacteria may seem much smaller and simpler—yet, their way of living provides insights into fundamental metabolic processes that have implications far greater than their size suggests. While the vast majority of bacteria that cluster in the human body are necessary for our existence and survival, certain types of bacteria are disease-causing organisms, or pathogens, and pose a danger to humans due to their ability to infect our cells.
How do bacteria affect humans?
Though bacteria are unicellular, by no means do they exist independently of one another. Rather, they exist in large communities that provide them with the strength needed to have a strong presence in even the most extreme environments.
Bacteria that naturally reside within humans have a commensal or mutualistic relationship with us: the human body provides the bacteria a place to thrive, and the bacteria prevent harmful pathogens from taking up space in our bodies, or aid in bodily functions, such as metabolism of nutrients and production of vitamins in the digestive tract.
However, parasitic bacteria harm the humans in which they reside, multiplying rapidly at the expense of their hosts. These parasitic bacteria mainly cause disease by secreting poisons known as toxins. Toxins may not only affect the body at the site of bacterial infection, but also at distant sites, due to the transport of toxins in the blood. Protein toxins are known to be highly lethal to humans—even more so than snake venom—and may act on specific cell types, such as the nerves or intestinal tract. For example, Clostridium tetani release a toxin known as tetanospasmin that attacks the motor neurons and causes muscle stiffness and spasms, symptoms of tetanus . Clostridium botulinum, the bacteria responsible for botulism, a type of food poisoning, also release toxins that target the neurons and may lead to paralysis. Other types of bacteria release toxins that are non- specific and lead to the death of many cell types in the body.
Fortunately, protein toxins are very unstable and lose their harmful characteristics over time . However, even when they lose their toxicity, they maintain their antigenic properties, meaning that they may still be detected by the immune system. By treating toxins with certain chemicals in the lab, scientists can artificially detoxify the toxins and use their antigenic properties to generate vaccines against certain bacteria. This mode of immunization has been used to prevent tetanus and diphtheria, which is caused by Corynebacterium diphtheriae and affects the mucous membranes of the nose and throat. Antitoxins, which are antibodies against toxins, may be used to treat these acute medical infections.
In addition to toxigenesis, invasiveness—or the ability of a pathogen to invade tissues—also influences the ability of bacteria to cause disease. A bacterium’s invasiveness depends on its mechanisms for colonization, which includes how it enters the body and proliferates, as well as its production of chemicals that directly promote tissue invasion. Colonization usually occurs in tissues that are indirect contact with the outside environment, such as the skin and mucous membranes, and involves the attachment of ligands on bacterial cells, called adhesins, to receptors on the surface of the host’s cells. These adhesin-receptor interactions may be very specific, as previous studies have shown that certain types of bacteria preferentially colonize certain types of tissue.
Bacterial adhesins may be either protein- or carbohydrate-based. Protein adhesins include fimbrialadhesins, which are hair-like structures also known as “pili,” and afimbrial adhesins, which do not form long protrusions, but instead are involved in closer contact with the host cell. Carbohydrate – based adhesins are usually a part of the bacteria’s cell wall, cell membrane, or capsule . Pathogenic bacteria may utilize combinations of protein – and carbohydrate-based adhesins to maximize their colonization of the human body. After adhesion, pathogenic bacteria are unable to be flushed out of the body by forces such as coughing, sneezing, or even mucosal shedding, and may begin to proliferate, secrete toxins, and further invade the host.
Pathogenic bacteria may either invade extracellularly or intracellularly. As the name suggests, extracellular invasion does not involve entering the host cells; however, pathogenic bacteria may still cause significant harm by secreting enzymes that degrade molecules, such as DNA or lipids, within host cells and inside host tissues. By degrading tissue barriers, pathogens may travel throughout the body to affect sites far away from the initial entry point. Intracellular invasion includes the entrance of pathogenic bacteria into the host cells. In some mechanism, engagement of bacterial surface proteins with host proteins induce cytoskeleton and membrane rearrangements, leading to the internalization of the bacterium. Pathogenic bacteria that utilize intracellular invasion either require the host cell to grow, or treat the host cell as a means by which to spread more quickly throughout the body.
Bacterial genome and its pathogenicity
What makes some bacteria pathogenic, and others harmless? While many factors such as the host cell’s genome or the environment play a role, the bacterial genome itself is crucial in determining its encoding of pathogenicity. Certain bacteria possess virulence genes in their genome, in regions of DNA known as “pathogenicity islands,” that code for virulence factors, including toxins, toxin-delivery proteins, or proteins that promote bacterial colonization . These virulence factors may also serve to promote bacterial evasion of the host’s immune response, as well as to inhibit or suppress the host’s immune system . Virulence genes may be transferred to the bacterium using plasmids or viruses by way of horizontal gene transfer. The latter process is important in the conversion of originally benign bacteria into harmful pathogens, as well as in the spread of antibiotic resistance in bacteria colonies.
Through conjugation, bacteria with tubular pili directly transfer pieces of their genetic material to other bacteria. Donor bacteria possess a plasmid that contains the necessary genes to build the conjugative bridge for “mating”with other bacteria, as well as to transfer DNA across it. In conjugation, the donor bacterium first binds to a recipient bacterium using a conjugation pilus, which pulls the two bacteria together. The pilus becomes a bridge through which a copy of a subset of the donor’s genes, such as the pathogenicity island, may be transferred to the recipient. In this way, resistance plasmids may be spread throughout a bacteria population , leading to antibiotic resistance.
Transduction is the process through which a bacteriophage, or a virus that infects bacteria, accidentally facilitates the transfer of DNA from one bacterium to another. Usually, when a bacteriophage infects a bacterium, it injects its viral genetic material into the bacterium, using bacterial machinery to transcribe and translate viral genes to express proteins necessary for its own reproduction, including those that comprise its protein coat. During this viral reproduction cycle, the protein coat may accidentally encapsulate a fragment of the host bacterium’s DNA, rather than viral DNA, creating a new “phony” bacteriophage. When this new phage is released and infects another bacterium, bacterial (rather than viral) DNA will be delivered to the new host and integrated into its genome via homologous recombination.
Conjugation and transduction may have also played a large role in allowing bacteria to be able to survive and thrive evolutionarily. Bacteria grow and divide asexually through “binary fission,”in which the original “parent” cell grows and splits itself into two identical “daughter” cells. This type of reproduction is associated with many benefits, including a lack of the need for a mate, and rapid population growth. (For example, some bacteria are able to double their population within a matter of minutes!) However, the lack of genetic diversity from binary fission means that the risk of death due to environmental factors for identical cells is very high. Through horizontal gene transfer, as well as through natural mutations, bacteria are able to diversify their genomes to allow for greater variation and an increased chance of survival.
What are examples of common pathogenic bacteria and their associated diseases?
In addition to those that cause tetanus, botulism, and diphtheria described above, there are many other types of pathogenic bacteria. Mycobacterium tuberculosis are rod- shaped bacteria that cause tuberculosis (TB), a respiratory disease that kills millions of people each year . TB is spread when a person with TB coughs or sneezes, releasing bacteria into the air. Streptococcus pneumoniae are the main type of bacteria responsible for pneumonia—another respiratory disease . In pneumonia, fluid or pus fills the alveoli of the lungs and causes difficulty breathing. As with TB, the main mode of transmission is through person-to-person contact by way of droplets formed when coughing or sneezing. Streptococcus pneumoniae may also lead to other infectious diseases, such as meningitis, an inflammation of the covering of the brain and spinal cord that may lead to brain damage.
You may have also heard of Salmonella, which may cause food poisoning and gastrointestinal disorders from ingestion of contaminated foods that are raw or insufficiently cooked. For example, consumption of raw eggs may cause food poisoning because of the presence of Salmonella in poultry or because of contamination of egg shells from poultry droppings. Different strains of Salmonella may cause typhoid fever, a potentially deadly disease that may lead to an overwhelming immune response and septic shock. Typhoidal Salmonella is usually spread from person to person. Shigella is also a major cause of food-borne illness and may cause dysentery and extreme discomfort in the gastrointestinal tract.
Other types of bacteria are normally harmless to the human body, but may cause disease under certain circumstances. For example, Staphylococcus aureus are found on the skin and in the nose of around one quarter of healthy adults, but may invade deeper into the body following skin damage . These infections that may affect the heart, lungs, and bones may turn life-threatening if improperly treated. Additionally, Escherichia coli are bacteria that normally reside in the gastrointestinal tract and keep it healthy . However, some subsets of this bacteria, like the Shiga toxin- producing strain, may cause food- borne illnesses, and others, such as the O157:H7 strain, may even cause acute kidney failure. Other notable pathogenic bacteria are Streptococcus pyogenes (which causes strep throat), Borrelia burgdorferi (which causes Lyme disease), and Treponema pallidum (which causes syphilis).
All in all, bacteria may seem like simple organisms, yet they possess a strong pathogenic potential that poses a danger to humans. However, scientific progress has allowed us to identify and fight against these different pathogenic bacteria . While many challenges must still be overcome, research efforts, medical advancements, and even simple hygiene efforts on your part may help to reduce the harmful effects of pathogenic bacteria to create a healthier world!
