I. THE INNATE IMMUNE SYSTEM

C. IMMEDIATE INNATE IMMUNITY

1. ANTIMICROBIAL ENZYMES AND PEPTIDES

Fundamental Statements for this Learning Object:

1. Immediate innate immunity begins 0-4 hours after exposure to an infectious agent and involves the action of soluble preformed antimicrobial molecules that circulate in the blood and are found in extracellular tissue fluids.
2. Lysozyme, found in in tears, mucous, saliva, plasma, tissue fluid, etc., breaks down peptidoglycan  in bacteria causing osmotic lysis.
3. Phospholipase A2 is an enzyme that penetrates the bacterial cell wall and hydrolyzes the phospholipids in the bacterial cytoplasmic membrane.
4. Human defensins are short cationic peptides 30-40 amino acids long that are directly toxic by disrupting the cytoplasmic membrane of a variety of microorganisms causing leakage of cellular needs.
5. Cathelicidins are proteins produced by skin and mucosal epithelial cells that are directly toxic to a variety of microorganisms.
6. Lactoferrin and transferrin, found in body secretions, plasma, and tissue fluid, trap iron for use by human cells while preventing its use by microorganisms.

 

LEARNING OBJECTIVES FOR THIS SECTION

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Innate immunity is an antigen-nonspecific defense mechanisms that a host uses immediately or within several hours after exposure to almost any microbe. This is the immunity one is born with and is the initial response by the body to eliminate microbes and prevent infection. Innate immunity can be divided into immediate innate immunity and early induced innate immunity. In this section we will learn about immediate innate immunity.

C. Immediate Innate Immunity

Immediate innate immunity begins 0-4 hours after exposure to an infectious agent and involves the action of soluble preformed antimicrobial molecules that circulate in the blood, our found in extracellular tissue fluids, and are secreted by epithelial cells. These include:

• antimicrobial enzymes and peptides;
• complement system proteins; and

These preformed antimicrobial molecules are designed to immediately begin to remove infectious agents as soon as they enter the body.

In addition to preformed antimicrobial molecules, the following also play a role in immediate innate immunity:

• anatomical barriers to infection
• mechanical removal of microbes
• bacterial antagonism by the body's normal microbiota

In this section we will look at how antimicrobial enzymes and peptides function to remove infectious agents.

1. Antimicrobial Enzymes and Antimicrobial Peptides

Examples include:

a. Lysozyme (def), found in in tears, mucous, saliva, plasma (def), tissue fluid, etc., breaks down peptidoglycan (def) in bacteria causing osmotic lysis. Specifically, it breaks the bond between the N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), the two sugars that make up the backbone of peptidoglycan (see Fig. 1).

b. Phospholipase A₂ (def) is an enzyme that penetrates the bacterial cell wall and hydrolyzes the phospholipids in the bacterial cytoplasmic membrane.

c. Human defensins (def)) are short cationic peptides 30-40 amino acids long that are directly toxic by disrupting the cytoplasmic membrane of a variety of microorganisms causing leakage of cellular needs (see Fig. 2). They also activate cells for an inflammatory response. Defensins are produced by leukocytes, epithelial cells, and other cells. They are also found in blood plasma and mucus. Certain defensins also disrupt the envelopes of some viruses.

d. Cathelicidins (def) are proteins produced by skin and mucosal epithelial cells. The two peptides produced upon cleavage of the cathelicidin are directly toxic to a variety of microorganisms. One pepitide also can bind to and neutralize LPS from Gram-negative cell walls to reduce inflammation.

e. Lactic and fatty acids, found in perspiration and sebaceous secretions (def), inhibit microbes on the skin.

f. Lactoferrin (def) and transferrin (def), found in body secretions, plasma, and tissue fluid, trap iron for use by human cells while preventing its use by microorganisms.

g. Hydrochloric acid and enzymes found in gastric secretions destroy microbes that are swallowed.

 

Keep in mind that in Unit 3 under "Virulence Factors that Promote Bacterial Colonization of the Host" we learned several mechanisms that various bacteria use to resist the body's antibacterial peptides. By resisting these immediate innate immune defenses, some bacteria have a better chance of colonizing their host.

Concept Map for Antibacterial Enzymes and Peptides

 

 

• QUIZ YOURSELF ON THIS SECTION

Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.