This document discusses and compares pathogenic and non-pathogenic microorganisms. It notes that pathogenic microorganisms can cause disease, while non-pathogenic ones generally do not. It provides examples of bacteria and the diseases they cause. The document also discusses opportunistic pathogens that may cause infection when the immune system is compromised. Finally, it outlines some factors that determine the pathogenicity and virulence of different bacteria.

1. Classificationof pathogenicand
non pathogenicmicroorganisms
Saajida Sultaana Mahusook

2. • Pathogenic and nonpathogenic bacteria are the two main
types of bacteria other organisms are in contact with.
• The distinction between the two can be made based on
the Koch’s Postulates.

3. Non-Pathogenic Microorganisms:
• Non-Pathogenic microorganisms are incapable of causing disease. i.e.,
harmless to other organisms
• They mostly live in the environment as saprophytes. Some of them are
autotrophs.
• Around 99% of bacteria are nonpathogenic.
• They are useful to man as they are involved in
• manufacturing foods such as butter, cheese,
• alcohol, lactic acid,
• Probiotics and fermented products
• antibiotics
• gradual decomposition of organic matter (dead animals, plants and their wastes) on
or in the soil.
• bio fertilizers etc.
• Some non-pathogenic bacteria live on the surface of animals as normal flora.
They are commensals.
• But, these bacteria can become opportunistic pathogens when they invade
the tissues. For example, E. coli are non-pathogenic bacteria that live in the
gastrointestinal tract and can trigger an immune response under certain
circumstances.

4. • It is estimated that the human body consists of approximately 1013
cells and hosts 1014–1015 individual micro-organisms.
• These micro-organisms can be divided into two groups:
• those that usually remain constant in their normal habitat (indigenous
or resident flora) and
• those that are accidentally acquired (transient flora)
• Body areas that usually harbor micro-organisms are skin, mouth,
nasopharynx, oropharynx and tonsils, large intestine and lower
ileum, external genitalia, anterior urethra, vagina, skin, and external
ear.
• Some functions of normal flora
• Protects the body from pathogenic microorganisms
• Keeps us healthy
• Synthesize certain vitamins etc.

5. Pathogenic Microorganisms:
• Pathogenic microorganisms are those that can cause disease.
• Pathogenicity is the capacity of a microorganism to cause a disease.
• The diseases caused by pathogenic bacteria are generally called infections.
• Pathogenic bacteria possess several genes that endow the capacity to cause
diseases.
• The agents that cause disease fall into five groups: viruses, bacteria, fungi,
protozoa, and helminths (worms).
Microorganism Disease
Bacteria Mycobacterium tuberculosis Tuberculosis
Bacillus anthracis Anthrax
Fungi Candida albicans Candidiasis
Tinea corporis Ringworm
Virus HIV (retro virus) AIDS
Herpes virus Herpes

6. Isenberg and D’ Amato classified microorganisms in disease production as
Murray et al. classified the pathogenicity of organisms as
Categories in both classifications are not always equivalent.
Opportunistic infections are defined as infection caused by non-
pathogenic microorganisms which become pathogenic when the immune
system is impaired by an unrelated disease. Example of an opportunistic
microorganism is Haemophilus ducreyi,which infects its host through broken
skin or epidermis.
commonly
involved
rarely
involved
occasionally
involved
High Potential Low

7. • Pathogenicity:
The ability of micro-organisms to induce disease, which may be assessed by disease-
carriage ratios
• Virulence:
The severity of the disease induced by micro-organisms. In epidemiological studies
virulence may be assessed by mortality or morbidity rates and the degree of
communicability
• Reservoir:
The place where the organism maintains its presence, metabolizes, and replicates
• Source:
The place from which the infectious agent passes to the host. In some cases the
reservoir and the source are the same, but not always
• Infection:
A microbiologically proven clinical diagnosis of inflammation
• Carriage:
Permanent (minimally 1 week) presence of the same strain in any concentration in body
sites normally not sterile (oropharynx, external nares, gut, vagina, skin)
• Abnormal carrier state:
The abnormal carrier state exists when the isolated micro-organisms is not a constituent
of normal flora (i.e., enterobacterial or pseudomonal strains)
• Colonization:
The presence of micro-organisms in an internal organ that is normally sterile (e.g., lower
airways, bladder). The diagnostic sample yields less than a predetermined level of
cfu/ml of diagnostic sample.

8. Factors that Determine the Pathogenicity of Bacteria
• Host susceptibility
Presence of mechanisms to fight with the infection by the
host immune system
• Pathogenic mechanisms of bacteria
Bacterial infectivity, host resistance, virulence genes, host-
mediated pathogenesis (Gram-negative bacterial sepsis,
tuberculosis, and tuberculoid leprosy) and intracellular growth
• Specific virulence factors
Adherence and colonization factors, invasion factors, presence
of a capsule and other surface components, endotoxins,
exotoxins, and siderophores

10. Reference
• https://pediaa.com/difference-between-pathogenic-and-
nonpathogenic-bacteria/
• Classification of Micro-Organisms According to Their Pathogenicity
M.A. DE LA CAL, E. CERDÀ, A. ABELLA, P. GARCIA-HIERRO
• https://www.ncbi.nlm.nih.gov/books/NBK8526/?report=classic
• https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Mic
robiology_(Boundless)/14%3A_Pathogenicity/14.2%3A_Overview_
of_Microbe-
Host_Interactions/14.2B%3A_Opportunistic_Microorganisms
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