Enzymes:

Serum lysozyme:

Provides innate & nonspecific immunity
Lysozyme is a hydrolytic enzyme capable of digesting bacterial cell walls containing peptidoglycan 
•    In the process of cell death, lysosomal NZs fxn mainly to aulolyse necrotic cells (NOT “mediate cell degradation”)
•    Attacks bacterial cells by breaking the bond between NAG and NAM.
•    Peptidoglycan – the rigid component of cell walls in most bacteria – not found in archaebacteria or eukaryotic cells
•    Lysozyme is found in serum, tears, saliva, egg whites & phagocytic cells protecting the host nonspecifically from microorganisms

Superoxide dismutase: catalyzes the destruction of O2 free radicals protecting O2-metabolizing cells against harmful effects 

Catalase:

- catalyzes the decomposition of H2O2 into H2O & O2
- Aerobic bacteria and facultative anaerobic w/ catalase are able to resist the effects of H2O2
- Anaerobic bacteria w/o catalase are sensitive to H2O2  (Peroxide), like Strep
- Anaerobic bacteria (obligate anaerobes) lack superoxide dismutase or catalase
- Staph makes catalase, where Strep does not have enough staff to make it

Coagulase

- Converts Fibronogen to fibrin
•    Coagulase test is the prime criterion for classifying a bug as Staph aureus – from other Staph species
•    Coagulase is important to the pathogenicity of S. aureus because it helps to establish the typical abscess lesion 
•    Coagulase also coats the surface w/ fibrin upon contact w/ blood, making it harder to phagocytize

NORMAL MICROBIAL FLORA 

A. Properties. Normal microbial flora describes the population of microorganisms that usually reside in the body. The microbiological flora can be defined as either 
1) Resident flora - A relatively fixed population that will repopulate if disturbed, 

2) Transient flora - that are derived from the local environment. These microbes usually reside in the body without invasion and can
even prevent infection by more pathogenic organisms, a phenomenon known as bacterial interference. 
The flora have commensal functions such as vitamin K synthesis. However, they may cause invasive disease in immunocompromised hosts or if displaced from their normal area. 

B. Location. Microbial flora differ in composition depending on their anatomical locations and microenvironments. The distribution of normal microbial flora.

Classification:

Neutrophiles (pH = 7.0)
- P. aeruginosaqo
- Clostridium sporogenes
- Proteus species

Acidophiles (pH < 7.0)
- Thiobacillus thiooxidans
- Sulfollobus acidocaldaarius
- Bacillus acidocaldarius

Alkaliphiles (pH > 7.0)
- Nitrobacter species
- Streptococcus pneumoniae

ANTIGENS

Immunogen
A substance that induces a specific immune response.

Antigen (Ag)
A substance that reacts with the products of a specific immune response.

Hapten
A substance that is non-immunogenic but which can react with the products of a specific immune response. Haptens are small molecules which could never induce an immune response when administered by themselves but which can when coupled to a carrier molecule. Free haptens, however, can react with products of the immune response after such products have been elicited. Haptens have the property of antigenicity but not immunogenicity.

Epitope or Antigenic Determinant
That portion of an antigen that combines with the products of a specific immune response.

Antibody (Ab)
A specific protein which is produced in response to an immunogen and which reacts with an antigen.

FACTORS INFLUENCING IMMUNOGENICITY

- Larger the molecule the more immunogenic it is likely to be.

- More complex the substance is chemically the more immunogenic it will be.

- Particulate antigens are more immunogenic than soluble ones and denatured antigens more immunogenic than the native form.

- Antigens that are easily phagocytosed are generally more immunogenic. This is because for most antigens (T-dependant antigens, see below) the development of an immune response requires that the antigen be phagocytosed, processed and presented to helper T cells by an antigen presenting cell (APC).

- Some substances are immunogenic in one species but not in another. Similarly, some substances are immunogenic in one individual but not in others (i.e. responders and non-responders). The species or individuals may lack or have altered genes that code for the receptors for antigen on B cells and T cells or they may not have the appropriate genes needed for the APC to present antigen to the helper T cells.

Method of Administration

1. Dose
The dose of administration of an immunogen can influence its immunogenicity. There is a dose of antigen above or below which the immune response will not be optimal.

2. Route
Generally the subcutaneous route is better than the intravenous or intragastric routes. The route of antigen administration can also alter the nature of the response

3. Adjuvants
Substances that can enhance the immune response to an immunogen are called adjuvants. The use of adjuvants, however, is often hampered by undesirable side effects such as fever and inflammation.

TYPES OF ANTIGENS

T-independent Antigens
T-independent antigens are antigens which can directly stimulate the B cells to produce antibody without the requirement for T cell help In general, polysaccharides are T-independent antigens. The responses to these antigens differ from the responses to other antigens.
Properties of T-independent antigens

1. Polymeric structure
These antigens are characterized by the same antigenic determinant .

2. Polyclonal activation of B cells
Many of these antigens can activate B cell clones specific for other antigens (polyclonal activation). T-independent antigens can be subdivided into Type 1 and Type 2 based on their ability to polyclonally activate B cells. Type 1 T-independent antigens are polyclonal activators while Type 2 are not.

3. Resistance to degradation
T-independent antigens are generally more resistant to degradation and thus they persist for longer periods of time and continue to stimulate the immune system.

T-dependent Antigens
T-dependent antigens are those that do not directly stimulate the production of antibody without the help of T cells. Proteins are T-dependent antigens. Structurally these antigens are characterized by a few copies of many different antigenic determinants as illustrated in the Figure 2.

HAPTEN-CARRIER CONJUGATES

Hapten-carrier conjugates are immunogenic molecules to which haptens have been covalently attached. The immunogenic molecule is called the carrier.

Structure
Structurally these conjugates are characterized by having native antigenic determinants of the carrier as well as new determinants created by the hapten (haptenic determinants). The actual determinant created by the hapten consists of the hapten and a few of the adjacent residues, although the antibody produced to the determinant will also react with free hapten. In such conjugates the type of carrier determines whether the response will be T-independent or T-dependent.

SUPERANTIGENS

When the immune system encounters a conventional T-dependent antigen, only a small fraction (1 in 104 -105) of the T cell population is able to recognize the antigen and become activated (monoclonal/oligoclonal response). However, there are some antigens which polyclonally activate a large fraction of the T cells (up to 25%). These antigens are called superantigens .

Examples of superantigens include: Staphylococcal enterotoxins (food poisoning), Staphylococcal toxic shock toxin (toxic shock syndrome), Staphylococcal exfoliating toxins (scalded skin syndrome) and Streptococcal pyrogenic exotoxins (shock).

Radioimmunoassays (RIA)

It is an extremely sensitive technique in which antibody or antigen is labelled with a radioactive material. The amount of radioactive material in the antigen-antibody complex can be measured with which concentration of antigen or antibody can be assayed. After the reaction ‘free’ and ‘bound’ fractions of antigen are separated and their radioactivity-measured.
 

THE PLASMIDS

The extrachromosomal genetic elements, called as plasmids are autonomously replicating , cyclic ,double stranded DNA molecules which are distinct from the cellular chromosome 

Classification

Plasmids can be broadly classified as conjugative and nonconjugative. 

Conjugative plasmids are large and self-transmissible i.e. they have an apparatus through which they can mediate their own transfer to another cell after coming in contact with the same. Example:  RF and certain bacteriocinogen plasmids.

Nonconjugative plasmids are small in size and can be mobilised for transfer into another cell only through the help of a conjugative plasmid. To this group belong some ‘r’ determinants and few bacteriocinogenic plasmids. Plasmids can also be transferred without cell contact by the process of transfection.

Properties of plasmids

Double stranded DNA , Autonomously replicate in host cell, Plasmd specific, Free DNA is transferred b transfection

Significance of Plasmids :The spread of resistance to antibiotics is one such well known example. These also play an important  role in the geochemical  cycle by spreading genes for the degradation of complex organic compounds.