Classification of Infections

Primary Infection- initial infection with a parasite in the host

Reinfection- subsequent infection by the same parasite

Secondary Infection- a new parasite infecting an IC host

Focal infection- infection at a localized site 

Cross infection- new infection est. From another host in a pt already suffering from ds

Nosocomial infections- HIC

Iatrogenic infection- through any therapeutic procedure

Causing an Infection

The endogenous – disease originates within the body.  Ex: metabolic disorders, congenital abnormalities, tumors.

The exogenous – disease originates outside the body.  Ex: chemical agents, electrical shock, trauma.

Nosocomial – acquired by an individual in a health care facility (workers to a patient). Many are antibiotic-resistant, life-threatening.

Opportunistic – occur when the body’s defenses are weak. Ex: pneumonia w/AIDs.

Source of Infection

• Humans from pt or carrier
• A healthy carrier
• Convalescent carrier
• Temporary carrier
• Contact carrier
• Paradoxical carrier
• Animals
• Insects vectors
• Mechanical vector
• Biological vector
• Soil and water 
• Food

Basic steps in Prevention of Infection

There are possible treatment and prevention to stop the infection cycle. This is through adequate hygiene, sanitary environment maintenance, and health education. 

Antimicrobial agents In Infection

• Anti-infective drugs such as antibiotics, antiviral, antifungal, and antitubercular drugs suppress infection.
• It can be administered by mouth, topically, or intravenously depending on the infection extent and severity.
• Sometimes, if drug resistance is known, multiple drugs are used to stop drug resistance and increase drug effectiveness. 
• Antibiotics only work for bacterial infection and have no effect on viral ones. 

History of infection control

1843 Oliver Wendell Holmes: contagious disease or communicable disease can be spread directly or indirectly from one person to another through contaminated hands. Ignaz Philipp Semmelweis observed a high mortality rate from MDs going from morgue to patients’ bedside without washing hands 1864 Joseph Lister: developed a surgical aseptic technique to prevent wound contamination.

Infection can be:

• Generalized or systemic (throughout the body)
• Localized (affecting one part of the body)

Signs and symptoms of infection:

• Systemic: headaches, fever, fatigue, vomiting, diarrhea, increased pulse and respiration
• Localized: redness, swelling, painful, warm to the touch

Chain of infection or Model of infectious disease transmission

Six elements must be present for an infection to develop

1.     The infectious agent 

2. Reservoir host

3. Portal of exit from the host

4.  Route of transmission

5.  Port of entry

6.  Susceptible host

1. Infectious agent: a pathogen must be present
2. Reservoir host: the pathogen must have a place to live and grow – the human body, contaminated water or food, animals, insects, birds, dead or decaying organic material. Humans who can transmit infection but how no signs of the disease are called carriers. A person may be unaware they are a carrier. 
3. Portal of exit: the pathogen must be able to escape from the reservoir host where it has been growing. Examples of portals of exit are blood, urine, feces, breaks in the skin, wound drainage, and body secretions like saliva, mucus, and reproductive fluids.
4. Route of transmission: When the pathogen leaves the reservoir host through the portal of exit, it must have a way of being transmitted to a new host. Examples of routes of transmission are air, food, insects, and direct contact with an infected person
5. Portal of entry: The pathogen must have a way of entering the new host. Common ports of entry are the mouth, nostrils, and breaks in the skin
6. Susceptible host: An individual who has a large number of pathogens invading the body or does not have adequate resistance to the invading pathogen will get the infectious disease

Breaking the chain of infection

Breaking at least one link stops the spread of infectious disease

1. The infectious agent - early recognition of signs of infection, Rapid, accurate identification of organisms 
2. Reservoir host - Medical asepsis, Standard precautions, Good employee health, Environmental sanitation, Disinfectant/sterilization
3. Portal of exit from the host, Medical asepsis, Personal protective equipment, handwashing, Control of excretions and secretions, Trash and waste disposal, Standard precautions 
4. Route of transmission - Standard precautions, Handwashing, Sterilization, Medical asepsis, Airflow control, Food handling, Transmission-based precautions
5. Portal of entry - Wound care, Catheter care, Medical asepsis, Standard precautions
6. Susceptible Host - Treating underlying diseases, Recognizing high-risk patients

Virulence and Pathogenicity

Pathogenicity: the capacity of microbes to cause disease

Virulence: the degree of pathogenicity of a specific microbe

Based on:

• Invasive qualities
• Toxic qualities
• Presence of pile or fimbriae for adhesion
• Ability to avoid host defenses (mutate)

Virulence Factors and Toxins

• Enzymatic Virulence Factors Examples: Coagulase (Staphylococcus aureus), Streptokinase (Streptococcus pyogenes), Hyaluronidase (Many pathogens), Collagenase (Many pathogens), Leukocidin (Many pathogens), Hemolysin  (Many pathogens)
• Adhesion Factors Examples: Protein A (Staphylococcus aureus), Protein M (Streptococcus pyogenes)
• Virulence factors help bacteria to, invade the host,  cause disease, and evade host defenses. 
• The following are types of virulence factors: 
• Adherence Factors: Many pathogenic bacteria colonize mucosal sites by using pili (fimbriae) to adhere to cells.
• Invasion Factors: Surface components that allow the bacterium to invade host cells can be encoded on plasmids, but more often are on the chromosome.
• Capsules: Many bacteria are surrounded by capsules that protect them from opsonization and phagocytosis
• Exotoxins - A type of bacterial toxin with the following properties: May be produced by either gram-positive or gram-negative bacteria, The action of the exotoxin does not necessarily require the presence of the bacteria in the host and Most exotoxins are peptide or protein 
• Most exotoxins are heat sensitive (exception: enterotoxin of Staphylococcus aureus)
• Exotoxins include several types of protein toxins and enzymes produced and/or secreted from pathogenic bacteria. Major categories include cytotoxins, neurotoxins, and enterotoxins.

Classes of exotoxins: Neurotoxic, cytotoxic, or enterotoxic exotoxins

• Neurotoxins: Interfere with proper synaptic transmissions in neurons
• Cytotoxins: Inhibit specific cellular activities, such as protein synthesis 
• Enterotoxins: Interfere with water reabsorption in the large intestine; irritate the lining of the gastrointestinal tract

Endotoxins

• A type of bacterial toxin having the following properties:
• Produced only by gram-negative bacteria 
• Endotoxins are a component of the gram-negative cell wall 
• The action of endotoxin requires the presence of the bacteria in the host. The endotoxin may be released from the cell wall as the cells die and disintegrate
• Endotoxin is composed of Lipid A: Part of the lipopolysaccharide layer 
• Mode of action: Irritation/inflammation of epithelium, GI irritation, capillary/blood vessel inflammation, hemorrhaging

• Endotoxins: The lipopolysaccharide endotoxins on Gram-negative bacteria cause fever, changes in blood pressure, inflammation, lethal shock, and many other toxic events.

Bacterial Identification Techniques

Accurate and definitive bacterial identification is essential for correct disease diagnosis, treatment of infection, and trace-back of disease outbreaks associated with microbial infections. Bacterial identification is used in a wide variety of applications including microbial forensics, criminal investigations, bio-terrorism threats, and environmental studies.

Techniques include:

• Conventional methods – Biochemical tests
• Antibody-based methods
• Nucleic acid-based methods – PCR, Southern blot, nucleic acid hybridization, RFLP, DNA fingerprinting
• Automated microbial identification methods
• DNA barcoding 
• Other methods – Phage typing, Flow cytometry, SDS

Bacterial Identification by Biochemical tests

Primary test

• Morphology
• Gram’s staining/Acid fastness
• Spores
• Motility
• The ability to grow in the air
• Ability to grow in the anaerobic conditions
• Catalase test
• Oxidase test
• Oxidation –Fermentation test

Secondary Tests

• Acetylmethylcarbinol production (VP) test
• Bile solubility test
• CAMP test
• Carbohydrate breakdown test
• Carbon sources test
• Chitinolytic test, Coagulase test
• Decarboxylase test
• Denitrification test
• Deoxyribonuclease test
• Gelatin hydrolysis test
• Haemolysin production test
• Hippurate hydrlysis test
• Hydrogen sulfide production test
• Indole test, Malonate test
• Methyl red (MR) test, O/129 sensitivity test
• ONPG test, Urease activity test
• Tween 20/80 hydrolysis test