Host-Microbe Interactions

The human body reacts in many different ways to microorganisms. These interac- tions can be summarized in the following categories:

RESIDENT MICROBIOTA

All surfaces of the human body, including the skin as well as the mucous membranes that surround the inner parts of the mouth, nostrils, genitals, and gastrointestinal tract, are inhabited by a distinct set of microbial communities, which are specifically adapted to the local physical and chemical environment. Such normal microorgan- isms, called resident microbiota, perform extremely important roles.

1. Resident microbiota engage all available binding sites on the host cell sur- faces, thus invaders have a diminished possibility of attaching to the host cell surfaces.

2. Many microbes secrete vitamins that are absorbed by the host and serve important nutritional needs.

3. Microbes also boost a form of generalized immunity against the pathogenic microorganisms that often share antigenic constituents with the resident microbiota.

4. In order to establish themselves in the host, the invading pathogen needs to establish itself and create a reservoir. Resident microbiota offer stiff competi- tion for nutrients and may produce secondary metabolites that will discour- age growth of invading pathogen.

Important Resident Microbiota

Skin

Staphylococcus epidermidis and species of Micrococcus, Corynebacterium, and Propionibacterium are the chief constituents of the resident microbiota on skin.

Certain yeast-like fungi, such as Candida, Malassezia, and Trichosporon spp., may also be found on the skin surfaces. However, any microorganism normally present in the environment can also be isolated from the skin. These may include Bacillus spp. and a wide range of fungal spores. But such microorganisms are not considered constituents of the resident microbiota; they are usually referred to as “transient microbiota.”

Nose and Throat

Staphylococcus epidermidis, Corynebacterium, Neisseria, and Haemophilus spp. are normally present in the nasal cavity and nasopharynx. Staphylococcus aureus can be isolated from the nose and external parts of the ears of nearly 50% of apparently healthy adults. The nose is the primary portal of entry for airborne (bioaerosol) diseases, such as tuberculosis, diphtheria, and influenza.

Mouth

The resident microbiota of the mouth include alpha hemolytic streptococci (Streptococcus spp.) and various species of Corynebacterium, Neisseria, Lactobacillus, and Candida. Also, a number of anaerobic bacteria are often present under the gums, just below the surfaces of the teeth. Microorganisms generally present in the air, food, and water can be also isolated from throat swabs, but they are not considered resident microbiota.

Gastrointestinal Tract

Various species of Bifidobacterium, Lactobacillus, Bacteroides, Klebsiella, Candida, Proteus, Enterococcus, and Escherichia, especially E. coli, are normally present in the intestine. In contrast, the stomach, due to its high acidity, offers a hostile envi- ronment to the microorganisms. A number of bacteria normally present in the intes- tine tract are also important opportunistic pathogens.

Vagina

Unlike the penis, which mostly harbors skin bacteria, the vagina affords a very dif- ferent environment. While its high acidity and the presence of antimicrobial sub- stances such as lactoferrin (an iron-binding protein) and S-IgA discourage the proliferation of microorganisms, the moist and often nutrient-rich conditions and partial to fully anaerobic environment favor the growth of certain microorganisms. Lactobacillus acidophilus is the main resident microorganism in the vaginas of women of child-bearing age. Vaginal microorganisms in prepubescent and post- menopausal stages are usually a mix of skin and intestinal microbiota. Other micro- organisms that may be isolated from the vagina include species of Streptococcus,

Bacteroides, and Candida, and coliform bacteria. Elderly women may also carry S. aureus. A disturbed vaginal environment can predispose the host to a wide range of infections caused by common gastrointestinal microorganisms.

HOST DEFENSES 

First Line of Defense

The uppermost layer of skin is comprised of dead, keratinized layer of cells. Thus, the intact skin is impervious to microorganisms, and functions as a powerful physical barrier. Most of the zoonotic diseases that are associated with insect bites result from the penetration of this barrier. Additionally, several soil and waterborne microorgan- isms can enter the human body through bruised or damaged skin.

Additionally, when the natural flow of the body’s fluids such as urine, bile, and saliva becomes blocked due to stone formation, this may cause a buildup of bacteria that may lead to infections such as urinary tract infection, cholecystitis, and parotitis.

Second Line of Defense (Chemical Secretions)

Low pH

Low pH generally discourages bacterial proliferation. Therefore, bacterial growth in the stomach (pH 1–3) and vagina (pH 4.5–5.5) is generally low.

Chemicals

Certain iron-binding proteins found in the body’s fluids discourage microbial growth by binding with free iron, which is essential for microbial growth. Some of the notable iron-binding proteins include lactoferrin (found in breast milk, semen, and the vagina) and transferrin (found in blood plasma). Lysozyme, an enzyme that dis- solves bacterial cell walls, is plentiful in tears and saliva. In addition, many body secretions, such as saliva, vaginal fluids, and breast milk, contain large quantities of a secretory immunoglobulin called S-IgA or IgA2, which tends to agglutinate and immobilize invading pathogens.

Third Line of Defense (Nonspecific Immunity)

Phagocytosis

Phagocytosis by neutrophils is perhaps the most active frontier in nonspecific immunity. Neutrophils patrol the entire body and engulf and destroy nonself particles (viruses, bacteria, yeasts, etc.). Similarly activated monocytes, called macrophages, may roam the tissues or gather at the site of infection and engulf the invaders. The macrophages in the liver are called Kupffer cells and dendritic cells, those in the lungs are called alveolar macrophages, and those in the kidneys intraglomerular mesangial cells. The engulfing of nonself entities occurs through a process called endocytosis; the engulfed microorganism is contained in a vacuole called phagolyso- some. The microbial destruction in the phagolysosome may be oxygen independent or oxygen dependent. The oxygen independent mechanism involves lysozyme, lac- toferrin, and defensins. The oxygen dependent mechanism kills through the produc- tion of NADPH (reduced nicotinamide adenine dinucleotide with an extra phosphate) oxidase, myeloperoxidase, nitric oxide synthetase, hydrogen peroxide, and hydroxyl radicals. 

Natural Killer (NK) Cells 

NK cells are a kind of lymphocyte that is different from T- and B-lymphocytes that participate in the specific immunity. Like macrophages, NK cells too roam all over the body and seek out and destroy nonself entities that have managed to get into the bloodstream. They recognize self from nonself by sensing changes in cell surface proteins that often result from microbial infections. Malignant cells are recognized in the same manner. The process may involve a major histocompatibility complex protein (MHC I), which is found on the surface of most nucleated cells. The NK cells have specific receptors that bind to MHC I protein, if the cell is from the self (host). If the NK cell is unable to bind with the cell, it is treated as nonself. The NK cells seem to “inject” a protein called perforin, which causes lysis of the infected cells and tumor cells. The NK cells also contain Fc receptors on their surfaces. The Fc receptors can bind with the infected host cells and destroy them directly by inject- ing perforin. 

Complements 

Complements, a set of interacting serum proteins, are also components of nonspe- cific immunity. There are nearly 20 different types of complement components, named C1, C2, C3, C4, and so on. Their mechanisms of action may involve a classic pathway wherein the complements act on the antigen–antibody complex to destroy the pathogen by injecting perforin in the invading cells. The perforin causes holes to form in microbial cells, resulting in their death. Another pathway called alternate pathway can destroy pathogens before a specific immune response. This pathway is initiated in response to certain bacterial molecules such as lipopolysaccharides (LPS) found in the cell walls of Gram-negative bacteria. Complement C3 is cleaved into C3a and C3b, which bind with the LPS leading to the formation of an enzyme called C3bBb. Further interactions with blood protein properdin and other complements lead to the formation of a membrane attack complex (MAC), which creates holes in the plasma membrane of the pathogen, resulting in cell death 

Fourth Line of Defense (Specific Immunity) 

Specific immunity is characterized by the presence of a memory system through which components of the immune system “remember” the pathogen and mount a massive attack each time the pathogen enters the body. Before any further discussion, certain related terms merit elucidation. These are cytokines and antigen. Cytokines are glycoproteins produced by numerous cells and they play an important role in specific as well as nonspecific immunity. They include interleukins (IL-1, IL-2, IL-3, etc.), interferons (IFN α, β, and IFN γ), and tumor necrosis factor (TNFα and TNFβ). The function of interleukins includes (but is not limited to) stimulation of T-cell proliferation, production and differentiation of macrophages, and growth and activa- tion of B cells, in addition to an effect on the central nervous and endocrine systems. Properties of interferons include antiviral and antiproliferative activities and stimula- tion of T-cells and macrophage and NK cells. The TNFs are cytotoxic to tumor cells and have numerous other properties including mediating inflammation. 

As stated earlier, the immune system recognizes foreign bodies via its ability to differentiate between self and nonself using MHT proteins. Any foreign body that is recognized as nonself is an antigen. However, in reality these are mostly glyco- proteins and polysaccharide molecules commonly present on the microbial cell surfaces. Antigens have antigen determinants called epitomes, which bind to T-cell receptors or specific antibodies. A number of small organic molecules may not act as antigens by themselves but they can act as antigens if combined with a protein or similarly large molecule. Such antigens are called haptens. For example, penicillin by itself is not antigenic, but when it combines with certain serum proteins, it becomes a hapten and triggers a strong immune response in sensitive persons. 

There are two lines of specific immunity, humoral and cellular immunity. It must be stated here that immunology is a highly developed branch of science and readers who want to develop an in-depth understanding of the topic should consult any of the several scholarly books in this field. Humoral immunity is mediated by B cells (B-lymphocytes). B cells originate and mature in the bone marrow but differentiation occurs in the lymphoid tissue. Activation of B cells can be antigen specific or T-cell dependent. The antigen presented by the macrophages stimulates T-helper cells, which in turm produce numerous cytokines that act on the B cells. The excited B cell develops into memory cells which store antigen-related genetic information, and plasma cells, which secrete specific antibodies. Antibodies, also called immuno- globulins (Ig), are essentially glycoproteins that are divided into five classes, namely IgG, IgM, IgA, IgD, and IgE. The building block of these immunoglobulins consists of two heavy chains linked together by disulfide bonds. Each heavy chain is attached to a shorter light chain via a disulfide bond. Both the heavy and the light chains have a larger constant region and a short variable region, which function as the binding sites (Fig. 2.1). A set of two light and heavy chains is also called a monomer. The IgG is a monomer and accounts for more than 80% of total serum antibodies. They neutralize toxins, opsonize pathogenic cells, and activate complements. A rise in the titer of IgG is often associated with the recovery stage of infection. IgG can also cross placenta. The IgM is a pentamer formed by connecting five monomers via

Figure 2.1.    Diagram of a monomer showing light and heavy chains, constant and variable regions, and the binding sites (IgG, IgD, IgE, and IgA1 are monomers).

disulfide bonds and a J chain. The IgM is often the first immunoglobulin to appear in the case of infection, and the titer increases as the infection progresses. IgM molecules are a very effective agglutinator and account for 5–10% of total serum antibodies. IgA can be divided into two groups, IgA1 and IgA2. IgA1, a monomer, accounts for about 5% of total serum antibodies. IgA2, a dimer (two monomers joined together with a J chain) also known as secretory IgA or S-IgA, is found on the surface of most mucocutaneous tissues lining external surfaces such as the mouth, nasal cavity, and vagina. It is also abundant in breast milk. IgD, a monomer, accounts for less than 1% of total serum antibodies. They are found on B cell sur- faces and play a role in antigen recognition. IgE, also a monomer, is mostly associ- ated with allergy. It is present in serum in very small quantities, often accounting for less than 0.05% of total serum antibodies. IgE titers rise during allergic episodes. 

Cellular immunity, the other kind of specific immunity, is mediated by T cells (T lymphocytes). T cells also originate in bone marrow, differentiate in lymphoid tissues, but mature in the thymus. The plasma membrane surface of T cells has specific receptors called T cell receptors (TCRs) that bind with the antigens. Two distinct components, an alpha polypeptide chain and a beta polypeptide chain are present on the receptor site. Part of the alpha and beta chains extend into the cyto- plasm while some of their portions remain on the membrane surface. The T cells react to antigen fragments attached to the major histocompatibility complex (MHC) molecules. The eventual activation of T cells is prompted by specific molecular signaling from inside the cells. Activated T cells multiply to form more specialized subsets including T helper (T-h) and T cytotoxic (T-c) cells. In particular, T-h cells, also known as CD4+ cells, are activated by antigens presented by MHC-II molecules on the antigen presenting cells (APC). The activated T-h cells perform several func- tions including promoting T-cytotoxic cells, activating macrophages, and triggering production of cytokines interleukin-2, interferon gamma, and tumor necrosis factor alpha by mediating inflammation. T-h cells also stimulate antibody production and play a critical role in allergic response. The T-c cells, also called CD8+ cells, do the actual and direct destruction of the pathogen cells or host cells containing pathogens such as viruses or any other intracellular pathogen. Activation of T-c cells may occur when they interact with APC (generally the macrophages or dendritic cells). Acti- vated T-c cells destroy the target cells either via the perforin pathway or through the CD95 pathway. In the perforin pathway, the binding of T-c cells to target cells induces cytoplasmic granules to move to the part of the cytoplasmic membrane that is in contact with the target cell. The granules then fuse with the plasma membrane and release perforin and granzymes. Perforin polymerizes the target cell membrane to produce pores. The granzymes then enter through the pores and destroy the target cells via a process called programmed cell death. During the course of CD95 pathway, the activated T-c cells increase expression of Fas ligands, which are a specific protein. Fas ligands interact with the transmembrane Fas protein receptors found on the surface of the target cells. As a result, target cell apoptosis ensues. Two other subsets of T-cells are T suppressor (T-s) and T memory (T-m) cells. The T-s cells assist in the functioning of the T-h cells and are believed to play a role in the host’s allergic response.