The Principles of Etiology
If an organism has the potential to cause disease, it is considered a pathogen; yet, certain species are highly pathogenic, meaning they do so frequently, while others do so infrequently. Opportunistic pathogens are those that can infect immunocompromised persons with serious infections, although they seldom, if ever, cause illness in immunocompetent individuals. The body's typical flora frequently includes these opportunists. The ability of the organism to seize the chance presented by weakened host defenses to cause illness is the source of the term "opportunistic."
Virulence, which is determined by the quantity of organisms needed to induce disease, is a quantitative indicator of pathogenicity. The number of organisms required to kill half of the hosts is known as the 50% lethal dose (LD50), and the number required to infect half of the hosts is known as the 50% infectious dose (ID50).
Because fewer organisms are required to cause illness or death, those with a lower LD50 (or ID50) are considered more virulent than those with a higher LD50 (or ID50).
The infectious dosage of a pathogenic bacteria that is necessary to induce illness varies widely amongst them. For instance, both Shigella and Salmonella infect the gastrointestinal system and produce diarrhea, but Shigella's infectious dosage is fewer than 100 organisms, whereas Salmonella's infectious dose is greater than 100 organisms is roughly equivalent to 100,000 organisms. The virulence factors of bacteria, such as their ability to adhere well to mucous membranes through their pili, their production of endotoxins or exotoxins, the presence of a capsule that shields them from phagocytosis, and their resistance to different nonspecific host defenses like stomach acid, are the main determinants of their infectious dose.
The word "parasite" has two meanings. The phrase, as used in this chapter, describes the parasitic relationship that bacteria have with their host cells—that is, how the bacteria's existence harms the host cells. Therefore, it is possible to think of bacteria that are human pathogens as parasites. Certain bacterial infections, like Rickettsia and Chlamydia, are obligatory intracellular parasites.
WHAT CAUSES PEOPLE TO GET CONTAGIOUS DISEASES?
When microbes overwhelm our host's defenses, or when the organism-host balance tips in favor of the organism, infectious diseases can infect humans. The organism or its byproducts are then present in sufficient quantities to cause a variety of symptoms, which we mistakenly identify as signs of an infectious condition, like fever and inflammation.
According to the organism, the quantity and virulence of the organisms exposed to the host, or person, are the two main factors in determining their ability to overwhelm the host.
The chance of infection increases with the number of organisms.
The sickness that the bacteria cause is also determined by the creation of particular virulence factors. For instance, bloody diarrhea is caused by a different strain of Escherichia coli that generates a different type of exotoxin, whereas watery (nonbloody) diarrhea is caused by a separate strain of E. coli that produces a different type of exotoxin. This chapter provides some noteworthy instances of particular diseases that are associated with the generation of different virulence factors.
Innate immunity and acquired immunity, which encompasses both cell- and antibody-mediated immunity, are the two primary components of the host's defense mechanisms. Any decrease in the efficiency of our host defenses tips the scales in favor of the organism and raises the likelihood of an infectious disease.
BACTERIAL INFECTION TYPES
There are multiple definitions for the word infection. One interpretation is that the individual has become infected by an organism, meaning it has invaded their body. For instance, a person infected with a low pathogenicity organism may not exhibit any symptoms of illness. When someone says, "I have an infection," they are referring to an infectious disease. This is another meaning of the term infection. Although the terms infection and disease are being used synonymously in this context, it's crucial to remember that the first definition does not require the terms to be synonymous with one another.
There are two main ways that bacteria cause disease: (1) they produce toxins, and (2) they invade and inflame. Exotoxins and endotoxins are the two main types of toxins. Endotoxins are lipopolysaccharides (LPS), which are an essential component of the cell wall, while exotoxins are polypeptides produced by the cell. Only found in gram-negative rods and cocci, endotoxins are characterized by fever, shock, and other widespread symptoms. They are not actively discharged from the cell. The presence of the bacteria in the host is not necessary for the production of symptoms by either exotoxins or endotoxins on their own.
Nonetheless, invasive bacteria proliferate rapidly in the local environment, causing erythema, edema, warmth, and pain as a result of the inflammation they cause. "Determinants of Bacterial Pathogenesis," which is the part that follows, goes into further detail about invasion and inflammation.
Not all illnesses are transmissible, meaning they can move from one host to another, but many can. For instance, the exotoxin that the organism in infected food produces when a person coughs spreads the disease from person to person. However, the exotoxin that causes botulism only affects the people who consume the contaminated food. An illness is referred to as contagious if it is highly transmissible.
An infection is classified as a pandemic if it spreads throughout the entire world, and as an epidemic if it happens far more frequently than usual. An endemic infection persists in a particular community at a low level throughout time.
Many infections are invisible or subclinical, and in addition to those that cause obvious symptoms, they can also be identified by isolating the organism or exhibiting an increase in antibody titer. Certain infections cause a latent condition, which may be followed by an organism's growth being reactivated and a recurrence of symptoms. A chronic carrier condition is caused by certain other infections when the organisms continue to develop in the host with or without symptoms. Chronic carriers, such as "Typhoid Mary," pose a risk to the public's health since they are a major source of infection for others.
Understanding two phenomena, normal flora, and colonization, is necessary to determine whether an organism collected from a patient is the true cause of the illness.
Periods of bacterial pathogen development
The majority of bacterial illnesses are contracted from outside sources. Certain bacterial illnesses, on the other hand, are not directly transferred before the commencement of infection because they are caused by members of the normal flora.
The following is a general order of the infection stages:
(1) Transmission into the entry gateway from an outside source.
(2) The primary host defenses, like stomach acid or skin, are evaded.
(3) Adherence, typically by bacterial pili, to mucous membranes.
(4) Colonization through bacterial growth at the adhesion point
(5) Inflammatory signs of disease brought on by the generation of toxins or their invasion.
(6) Host reactions in phases 3, 4, and 5, both nonspecific and specific (immunity).
(7) The disease's course and outcome.
Factors Influencing Bacterial Pathogenicity
1. Distribution
From the standpoint of public health, it is crucial to comprehend how bacteria and other infectious agents spread because breaking the chain of transmission is a great method to stop the spread of infectious diseases. Infectious diseases can spread from nonhuman sources such as soil, water, and animals in addition to the common "human-to-human" route of transmission. Fomites are inanimate items that harbor bacteria that can lead to infectious diseases. Examples of these objects are towels. A few significant instances of these transmission modalities
While certain illnesses are brought on by individuals found in the normal flora, the majority are contracted by external transmission. Additionally, sexual contact, urine, skin contact, blood transfusions, contaminated needles, and insect bites can all spread the spread of organisms. Different bacterial and viral diseases can spread through blood transfusions or shared needles during intravenous drug use. The risk of infection has been significantly decreased by testing donated blood for Treponema pallidum, HIV, human T-cell lymphotropic virus, hepatitis B, hepatitis C, and West Nile virus.
In the US, the most common bacterial illnesses spread by ticks are tularemia, ehrlichiosis, relapsing fever, Lyme disease, and Rocky Mountain spotted fever. The most prevalent of these five illnesses is definitely Lyme disease. Three infectious diseases are spread by ticks in the genus Ixodes: ehrlichiosis, babesiosis, and Lyme disease. Babesiosis is a protozoan disease.
Another way that microbes can spread from mother to child is through vertical transmission, which involves bacteria, viruses, and other microorganisms. There are three ways that organisms might spread vertically: through breast milk, through the placenta, and through the birth canal. A few major therapeutically relevant vertically transmitted organisms. (In contrast, horizontal transmission is a person-to-person transfer that does not occur from mother to child.)
2. Stickiness on Cell Surfaces
Some bacteria create compounds (like capsules or glycocalyces) or have specific structures (like pili) that enable them to stick to the surface of human cells, increasing their capacity to cause disease. For organisms to adhere to mucous membranes, certain adherence processes are necessary; mutants lacking these mechanisms are frequently not harmful. For instance, the glycocalyx of Staphylococcus epidermidis and several viridans streptococci enables the organisms to firmly cling to the endothelium of heart valves.
Following attachment, the bacteria frequently create a biofilm, which is a protective matrix made of several proteins and polysaccharides. In addition to natural structures like heart valves, biofilms also develop on foreign bodies including prosthetic joints, prosthetic heart valves, and intravenous catheters. Antibiotics and host immunological responses like antibodies and neutrophils are both thwarted by biofilms against bacteria. Additionally, especially in diabetics, they impede wound healing, leading to persistent infections in the wounds. Dental plaque, which is the first step towards dental cavities, and Pseudomonas persistence in the lungs of cystic fibrosis patients are both significantly influenced by biofilms.
Quorum sensing is the mechanism by which bacteria, like Pseudomonas, regulate the formation of biofilms. When a quorum is sensed—that is, when a specific bacterial density is reached—the bacteria in quorum sensing multiply in a non-aggressive manner. At that moment, new bacterial virulence factors—like biofilms, for example—that aid in pathogenesis are synthesized.
Artificial joints and heart valves are examples of foreign bodies that increase the risk of infection. These surfaces allow bacteria to stick, but phagocytes don't stick as well since the artificial surface lacks selectins and other binding proteins.
