1. Microbial Interactions32
Microbial Interactions
Copyright © McGraw-Hill Global Education Holdings, LLC. Permission required for reproduction or display.

2. Microbial Interactions…
Symbiosis = an association of two or more different species of organisms
relationships can be intermittent and cyclic or permanent
Types of interactions include
mutualism, cooperation, predation, commensalism, parasitism, amensalism, and competition

3. Mutualism Some reciprocal benefit to both partners
Relationship with some degree of obligation
partners cannot live separately
Mutualist and host are dependent on each other

5. Commensalism
One organism benefits and the other is neither harmed nor helped (neutral)
Commensal - organism that benefits
Often syntrophic - growth of one organism depends on or is improved by growth factors, nutrients, or substrates provided by another organism growing nearby
Can also involve modification of environment by one organism, making it more suited for another organism

6. Examples of Commensalism
Microbial succession during spoilage of milk
fermenting bacteria promote growth of acid tolerant species
Formation of biofilms
initial colonizer helps other microorganisms attach
Skin or surface microbes on plants or animals
host plant or animal releases volatile, soluble, and particulate organic compounds used by commensals

7. Cooperation
Like commensalism, a positive (not obligate) symbiosis which involves syntrophic (one organism lives off the byproducts of another) relationships
Benefits both organisms in relationship
Differs from mutualism because cooperative relationship is not obligatory

8. Predation Parasitism
Among microbes involves a predator species that attacks, usually killing its prey
Bdellovibrio penetrates cell wall, grows outside plasma membrane
Benefits by providing nutrients for primary producers
One organism gains (parasite) and the other is harmed (host)
Always some co- existence between host and parasite
Successful parasites have evolved to co-exist in equilibrium with their hosts
if balance upset, host or parasite may die

9. Ammensalism
Negative impact of one organism on another based on release of a specific compound
Some examples
antibiotic production by fungi and bacteria
use of antibiotic-producing streptomycin by ants to control fungal parasites
bacteriocin production by bacteria
production of antibacterial peptides by insects and mammals
e.g., cecropins, defensins, and athelicidins
production of organic acids during fermentation

10. Competition
Occurs when two organisms try to acquire or use the same resource
Two possible outcomes of competition
one organism dominates
competitive exclusion principal
two organisms overlap too much in their resource use, and one population is excluded
two organisms share the resource
both survive at lower population levels

11. Human-Microbe Interactions
The human body is a diverse environment
specific niches are present
dynamic relationships exist
Microbiome
all the genes of the host and the microbiota
goal is to determine the impact that microbial gene function has on human health
Pathogenicity
ability to produce pathological change or disease
Pathogen
any disease-producing microorganism

12. Normal Microbiota of the Human Body
Normal microbiota or microflora
microbes regularly found at an anatomical site
Relationship begins at birth
varies with environment and food source
Bifidobacteria
found in breast fed babies
protrophic – can synthesize all amino acids and growth factors from simple carbohydrates

13. The Relationship between Normal Microbiota and the Host
Usually mutually beneficial
normal microbiota often prevent colonization by pathogens
bacterial produces, e.g., vitamins B and K are beneficial to the host
Opportunistic pathogens
members of normal microbiota that produce disease under certain circumstances
Compromised host
debilitated host with lowered resistance to infection

15. Skin Commensal microbes include both resident and transient microbiota
Mechanically strong barrier
Inhospitable environment
slightly acidic pH
high concentration of NaCl
many areas low in moisture
constant sloughing of skin cells
Inhibitory substances (e.g., lysozyme, cathelicidins)

16. Acne Vulgaris Caused in part by activities of Propionibacterium acnes
sebum
fluid secreted by oil glands
accumulates, providing hospitable environment for P. acnes
comedo
plug of sebum and keratin in duct of oil gland
results from inflammatory response to sebum accumulation

17. Nose and Nasopharynx Staphylococcus aureus and S. epidermidis
predominant bacteria present
found just inside nostrils
Nasopharynx may contain low numbers of potentially pathogenic microbes
e.g., Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae

18. Oropharynx
Division of the pharynx lying between the soft palate and the upper edge of the epiglottis
alpha-hemolytic streptococci
diphtheroids
Gram-negative cocci
anaerobes in tonsillar crypts

19. Respiratory Tract No normal microbiota Microbes moved by:
continuous stream of mucous generated by ciliated epithelial cells
phagocytic action of alveolar macrophages
lysozyme in mucus

20. Eye and External Ear Eye External ear
from birth throughout a human life, small numbers of bacterial commensals are found on the conjunctiva of the eye
the predominant bacterium is Staphylococcus epidermidis
External ear
similar to skin flora as well as fungi

21. Mouth
Contains organisms that survive mechanical removal by adhering to gums and teeth
contribute to formation of dental plaque, dental caries, gingivitis, and periodontal disease
Within hours of birth, the oral cavity is colonized by microorganisms from the surrounding environment

22. Stomach Small Intestine
Divided into three areas
duodenum
contains few organisms
jejunum
ileum
flora present becoming similar to that in colon
pH becomes more alkaline
Most microbes killed by acidic conditions
some survive if pass through stomach very quickly
some can survive if ingested in food particles

23. Large Intestine (Colon)
Largest microbial population of body
eliminated from body by peristalsis, desquamation, and movement of mucus
replaced rapidly because of their high reproductive rate
most of the microbes present are anaerobes
Bacteroides thetaiontaomicron
colonizes exfoliated host cells, food particles, and sloughed mucus

24. Genitourinary Tract Kidneys, ureter, and bladder
normally free of microbes
Distal portions of urethra
few microbes found
Female genital tract
complex microbiota in a state of flux due to menstrual cycle
acid-tolerant lactobacilli predominate