This document discusses the classification of pathogenic bacteria according to their morphological, anatomical, staining, environmental, and pathogenic properties. Bacteria can be classified into six main groups based on their cell shape and structure: cocci, bacilli, actinomycetes, spirochetes, mycoplasmas, and rickettsiae. Other classification schemes include whether bacteria are gram-positive or gram-negative, form spores or flagella, are acid-fast or not, and whether they are aerobic or anaerobic. Pathogenic bacteria are further classified based on if they cause disease, their relationship with the host, and their nutritional requirements.

1. Medical Microbiology Laboratory
(Pathogenic Bacteria Classification)
Hussein A. Abid
Medical Laboratory Scientist
Member at American Society of Microbiology
Chairman of Iraqi Medical Laboratory Association
Teacher at Middle Technical University

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BACTERIA
 Bacteria are living organisms that are classified as
prokaryotes.
 They lack membrane-enclosed nuclei and organelles,
i.e., mitochondria, Golgi complexes, endoplasmic
reticulum.
 They could be classified according:
1) Morphological properties 2) Anatomical properties 3) Staining reaction
4) Environmental factors 5) Pathogenicity 6) Nutrition
7) Relationship between host and organism

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MORPHOLOGICAL PROPERTIES
 Morphological properties include (cell shape, size, structure,
and arrangements).
 Bacteria can be classified into six groups according to their
morphological properties:
1. True bacteria: divided into two groups cocci and bacilli
2. Actinomycetes
3. Spirochetes
4. Mycoplasmas
5. Rickettsiae
6. Chlamydia

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MORPHOLOGICAL PROPERTIES
 Actinomycetes:
o These are rigid organisms like true bacteria
but they resemble fungi in that they exhibit
branching and tend to form filaments.
o They are termed such because of their
resemblance to sun rays when seen in
tissue sections.
 Spirochetes:
o These are relatively longer, slender, non-
branched microorganisms of spiral shape
having several coils.
Leptospira interrogans
Actinomycetes (Nocardioforms)

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MORPHOLOGICAL PROPERTIES
 Mycoplasmas:
o These bacteria lack in rigid cell wall (cell
wall lacking) and are highly pleomorphic
and of indefinite shape.
o They occur in round or oval bodies and in
interlacing filaments.
 Rickettsiae and chlamydia:
o These are very small, obligate parasites, and at one time were
considered closely related to the viruses. Now, these are regarded
as bacteria.

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ANATOMICAL FEATURES
 Morphological properties include
(capsule, flagella, and spores).
 According to capsule
1. Capsulated (Streptococcus pneumoniae)
2. Non-capsulated (Viridans streptococci)
 According to flagella
1. Flagellate (Monotrichous, Lopho-trichous,
Amphi-trichous and peritrichous)
2. Afflagellate (e.g. Shigella spp.)

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ANATOMICAL FEATURES
 According to sporulation
1. Spore forming (e.g. Bacillus spp.)
2. Non-spore forming (Escherichia coli)

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STAINING REACTIONS
 Gram’s stain reaction:
1. Gram positive (+ve) bacteria: after gram staining
procedure, the bacteria appear blue to purple in color.
2. Gram negative (-ve) bacteria: after gram staining
procedure, the bacteria appear red to pink in color.
 Acid-Fast stain reaction:
1. Acid-fast bacteria: after Ziehl-Neelsen staining procedure,
the bacteria will appear pink in color.
2. Non-acid-fast bacteria: Ziehl-Neelsen staining procedure,
the bacteria will appear blue in color.

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STAINING REACTION (examples)
 Gram’s stain reaction:
1. Gram positive (+ve) cocci: Staphylococcus aureus
2. Gram negative (-ve) cocci: Neisseria gonorrhoae
3. Gram positive (+ve) bacilli: Clostridium perfringens
4. Gram negative (-ve) bacilli: Pseudomonas aeroginosa
 Acid-Fast stain reaction:
1. Acid-fast bacteria: Mycobacterium tuberculosis
2. Non-acid-fast bacteria: Staphylococcus aureus

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PATHOGENICITY
 Pathogenic bacteria: Are able to cause disease (e.g.
Salmonella typhi).
 Non-pathogenic bacteria: Do not cause a disease (e.g.
lactobacilli)

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RELATIONSHIP BETWEEN HOST
AND ORGANISM
 Saprophytes: free living microbes on dead animals or
decaying things, that can be found in soil and water and play
important role in degradation of organic matter.
 Parasites: that establish themselves and multiply in hosts.
 Commensals: microbes that live in complete live with
harmony, but they can produce disease when host
resistance is lowered. (e.g. normal flora organism).

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NUTRITION
 Autotroph: organism that produces complex organic
compounds (such as carbohydrates, fats, and proteins) from
simple substances present in its surroundings.
 Heterotroph: organism that cannot produce its own food,
relying instead on the intake of nutrition from other sources
of organic carbon, mainly plant or animal matter.

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ENVIRONMENTAL FACTORS
1. Temperature
2. pH
3. Oxygen requirements
4. Living in salt
5. Atmospheric pressure

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TEMPERATURE
 Psychrophiles (15-20 ºC): Pseudomonas fluorescens
 Mesophiles (20-40 ºC): Escherichia coli, Staphylococcus
aureus
 Thermophiles (50-60 ºC)
 Extremely thermophiles (as high as 250 ºC)

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pH
 Acidophiles
 Alkaliphiles (e.g.Vibrio spp.)
 Neutralophiles, pH 6-8, majority of the medically
important bacteria grow best at neutral or slightly alkaline
media (pH 7.2-7.6).

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OXYGEN REQUIREMENTS
 Aerobes: grow in ambient temperature, which contains 21%
O2 and a small amount of CO2, 0.03%
 Obligate aerobes: Strictly require O2 for their growth
(Pseudomonas aeruginosa)
 Microaerophilic: grow under reduced O2, 5- 10% and
increased CO2, 8-10%; (e.g. Campylobacter jejuni,
Helicobacter pylori).
 Obligate anaerobes: doesn't need O2 for growth, may be
inhibited by oxygen.

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OTHER WAYS FOR CLASSIFICATION
 Motility: motile/non-motile
 Antibiotic sensitivity: sensitive/resistant
 Fermentation: fermenter/non-fermenter

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CLASSIFICATION OF PATHOGENIC BACTERIA
Gram Positive
AnaerobesAerobes
BacilliCocciBacilliCocci
Clostridium spp.Peptococcus spp.Corynebacterium spp.Staphylococcus spp.
Peptostreptococcus spp.Bacillus spp.Streptococcus spp.
Listeria spp.Enterococcus spp.
Actinimyces spp.
Nocardia spp.
Lactobacillus spp.
Erysipelothrix spp.

19. Gram Negative
AnaerobesAerobes
BacilliCocciBacilliCocci
Bacteroides spp.Veillonella spp.EnterobacteriaceaeNeisseria spp.
Fusobacterium spp.Pseudomonas spp.
Prevotella spp.Brucella spp.
Bordetella spp.
Francisella spp.
Legionella spp.
Chlamydia spp.
Bartonella spp.
Coccobacilli
Moraxella spp.
Haemophilus spp. 19
CLASSIFICATION OF PATHOGENIC BACTERIA

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CLASSIFICATION OF PATHOGENIC BACTERIA
Other bacteria
Mycobacterium spp.
Mycoplasma spp.
Rickettsia spp.
Treponema spp.
Borrelia spp.
Leptospira spp.

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CLASSIFICATION OF PATHOGENIC BACTERIA