Lecture 5 - Microbial Ecology

Microbial ecology Dr. Edet E. Udo Department of Microbiology Faculty of Medicine, Kuwait University.

Microbial ecology • • • •

Objectives: At the end of this session, students should: Define microbial ecology Discuss the different types of host- bacterial relationship • Discuss Normal flora and their nature and significance at various body sites • Discuss the habitats of common bacterial pathogens • Discuss biofilm and clinical significance of biofilms

Microbial ecology • Definition: • Ecology is the study of the relationship between organisms and their environment. • Types of relationships: • Organisms relate to each other in a variety of ways. • Host: an organism that harbours another organism. • Symbiosis: the living together in close association of two or more dissimilar organisms.

• Commensalism • ( eating at the same table): A relationship in which one organism is neither harmed nor helped while the other organism or population gains. It is a non obligatory relationship. • Commensal bacteria • live in or on the host and benefits from it but does not harm the host. • Mutualism: • a relationship in which both populations benefit from the association. The host and the mutualist are metabolically dependent. • Chemotaxis helps bring organisms together in a mutualistic relationship

Microbial ecology • Parasitism: • A relationship that benefits on organism at the expense of the other. • Parasitic relationship are often very specific e.g. Virus/host relationships. • In parasitic relationship, the parasite gets a suitable environment for growth. • The host provides the parasite nutritional needs either as food itself or the parasite uses the host food or metabolic product. • Parasitic organisms : (pathogens) bacteria, viruses, fungi, plants, animals.

Types of parasites • Ectoparasites: on the surface of the host • Endoparasite: parasite lives inside the host

Ectoparasites

Endoparasites

• Pathogens: – Microorganisms capable of causing disease • Pathogenic microorganisms • have special properties that allow them to invade the human body or produce toxins. • When the microorganisms overcomes the body’s defenses, a state of disease results • Disease: An abnormal state in which part or all of the body is not properly adjusted or is not capable of carrying out normal functions • Infection is the invasion and and growth of pathogens causing tissue injury.

Types of pathogens: • Primary pathogens : • always associated with disease. E.g. Mycobacterium tuberculosis, Neisseria gonorrhoeae • Opportunistic pathogens: • Parts of the normal flora but becomes pathogenic under certain situations. Examples include E. coli, staphylococci, Pseudomonas spp etc. • Non pathogens: Not known to cause disease

• Contamination: • the transient presence of microbes, pathogenic or nonpathogenic, on the body surfaces without any injury or invasion of tissues • Colonization: • Continuing presence of microbes for extended periods of time without causing injury or invasion of tissue

Normal flora • Definition: • Microorganisms that establish permanent colonies inside or on the body without producing disease. • They are frequently found in particular sites in normal healthy individuals. • The composition and numbers of the flora vary in different areas of the body and sometimes at different age.

Normal flora • Normal flora can prevent pathogens from causing an infection ( microbial antagonism) • Normal flora and the host exist in a symbiotic relationship • It is estimated that a typical human body contains 1 X 1013 cells yet harboura 1 X 1014 bacterial cells; the Large intestine contains > 100 billion bacteria

Normal flora The healthy fetus: • is sterile until birth., later exposed to the flora of the mother’s genital tract, to the skin and respiratory flora of those handling it, and to organisms in the environment • Sterile body sites ( without normal flora): • Blood, body fluids and tissues

• Transient flora • Composed of microbes that are present for various periods and then disappear. • Occasional bacteria may be present briefly in the bloodstream • Transient bacteremia may be the source of infections of damaged or abnormal valves that leads to Subacute bacterial endocarditis

Role of Normal flora • 1. Infection resistance – Occupy of binding sites on the surface of human body cells and prevents the attachment of pathogens to the same sites – Mucin secretions- prevents attachment of pathogenic organisms – Secretion of noxious substances that may be injurious to pathogenic bacteria • 2. Stimulate the immune system: – Normal flora, especially Gram-negative rods, are strong immunogens. – Because of cross-reactivity between related bacteria , the antibodies produced in response to commensal organisms confer protection to the host from many pathogenic organisms.

Role of Normal flora • 3. Act as nutrient source: – Members of the normal flora provide vitamin K, a cofactor essential for the synthesis of clotting factors II, VII, and XI. – Without this contribution from the normal flora, the human clotting system would be grossly ineffective • 4. Stimulate epithelial cell turnover. – Without normal flora, the gut would be thin and without tone.

• Normal flora as pathogens: • 1. Opportunistic infection by normal flora is a common problem • Immunocompromised host: – In patients immunocompromised by congenital defects, disease or therapy, a wide variety of harmless microorganisms can cause severe disease e.g AIDS patients. Antibacterial therapy: .Antibiotic- associated colitis caused by Clostridium difficile and mucosal infections caused by yeasts often follow antibiotic administration and disturbance of the normal flora.

– Trauma: accidental or surgical trauma. E.g. Postoperative abdominal abscesses and posttraumatic peritonitis. • 2. Bacterial translocation: – the passage of viable bacteria from the gastrointestinal tract through the epithelial mucosa into the systemic circulation • 3. Tissue invasion:. – E.g. Streptococcus mutants, causes dental caries by producing gelatinous deposits that allow acid-producing bacteria to adhere to the tooth enamel leading to the formation of caries.

Anatomic location of Normal flora

• Body sites

• Skin, external ear, distal urethra

• Mouth, Tongue and buccal mucosa

• Flora

• Propionibacterium acnes, Staphylococcus epidermidis, diphtheroids, Micrococcus spp, alphaand nonhaemolytic streptococci • Viridans streptococci, Neisseria species, Branhamella , occasional Candida albicans

• Nasopaharynx • (URT)

• Stomach • Small intestine

• Colon: Adults

• Oral organisms, transient carriage of Streptococcus pneumoniae, Haemophilus spp, Neisseria meningitidis, oral anaerobes. • Rapidly becomes sterile after meals • Scanty, variable and ill defined • Mostly anaerobes: Bacteriodes spp., Fusobacterium spp, Clostridium spp, Peptostreptococcus. Escherichia coli, Proteus, other Enterobactericeae, Pseudomonas aeruginosa, numerous other bacteria and yeast

• Colon: • During breast feeding • Vagina:

• Eye

• Bifidobacterium, lactobacilli, aciduric streptococci

• Lactobacilli, aciduric streptococc, anaerobes, yeasts

• Diptheroids (Corynebacterium xerosis), Staphylococcus epidermidis, non haemolytic streptococci, Haemophilus spp, Moraxella.

Concept of prebiotics and probiotics • The complex and balanced flora of the large intestine prevents infection and and maintains a positive effect on nutrition. • Abrupt changes in diet, stress or antibiotic therapy can upset the microbial balance and render the host susceptible to diseases • . • The activities of health promoting flora in the large intestine, preferably at the expense of pathogens- can be improved through dietary supplements.

Concept of prebiotics and probiotics

• Prebiotics • are dietary components that have a specific fermentation directed towards populations of gut bacteria that are thought to be of benefit to health. • Prebiotics • are defined as a non-digestible food ingredient that affects the host by selectively targeting the growth and / or activity of one or a limited number of bacteria in the colon and thus has the potential to improve host health. • Examples are oligofructose and inulin which, stimulates the growth of bifidobacteria in the

• Probiotics: • The oral administration of living microorganisms to promote health and growth. They consist of lactobacilli, bifidobacteria , streptococci etc. • Characteristics: • adhere to host intestinal mucosa, • are easily cultured, non pathogenic, non toxic, • produce useful enzymes or physiological end products that benefits the host, • remain viable for long periods and withstand stomach acid and bile salts.

• How probiotics work to maintain health (Mode of action): – They compete with pathogens for nutrients and adhesion sites – Inactivate pathogenic bacterial toxins or metabolites – Produce substances that inhibits pathogens’ growth, stimulate non specific immunity • Health benefits of probiotics: – They have anticarcinogenic activity, – control intestinal pathogens growth, – improve lactose use in individuals with lactose intolerance, – reduce serum cholesterol.

Microbial biofilms • Biofilms are communities of microbes associated with a surface such as pipes, soils, medical implants and epithelial cells • Biofilms can be composed of populations that develop from a single species or a community derived from multiple microbial species, and they can form on a vast array of abiotic and biotic surfaces • Biofilm formation may require coordination with, interactions of, and communication between multiple bacterial species. .

Biofilm on Intravenous Catheter Connecter 24 hours after Insertion

Characteristics of biofilms attached bacteria in disease • Clinical biofilms infections are characterized by symptoms that typically recur even after repeated treatments with antibiotics. • biofilms infections are rarely resolved by the host's immune system. • Biofilm bacteria release antigens and stimulate the production of antibodies, yet bacteria residing in biofilms are resistant to these defense mechanisms

Implant-Based ( Devise -associated) Infections • Bacterial biofilms on prosthetic valves are the leading cause of endocarditis in patients who have undergone heart valve replacement. • The mortality rate among this group of patients is as high as 70% • Millions of catheters (e.g., central line, intravenous, and urinary catheters) • are inserted into patients every year, and these implants serve as a potential surface for biofilm formation.

TABLE 1.   Examples of common implant infections Implant

Organism(s) found

Associated disease or consequences

Prosthetic valve Contact lenses Intravascular catheters

S. epidermidis, S. sanguis

S. epidermidis, S. aureus

Septicemia, endocardit is

Total artificial heart

P. aeruginosa, S. epidermidis, S. aureus

Septicemia, device fail ure

Urinary catheters

E. coli, P. aeruginosa, E. faecalis, Proteus mirabilis

Joint replacement

S. epidermidis, S. aureus

Septicemia, device fail ure

Endotracheal tube

P. aeruginosa, E. coli, S. epidermidis, S. aureus

Pneumonia

Voice prostheses

Streptococci staphylococci

P. aeruginosa, S. epidermidis

Prosthetic valve endoc arditis Keratitis

Bacteriuria

Prosthesis failure