Gram negative bacteria are identified by their inability to retain crystal violet stain using the Gram staining method. They appear pink under microscopy due to the counterstain. Gram negative bacteria have an outer membrane containing lipopolysaccharides and a thin peptidoglycan layer, distinguishing them from Gram positive bacteria which have a thick peptidoglycan layer but lack an outer membrane. Common Gram negative bacteria that can cause human diseases include Escherichia coli, Salmonella, Shigella, Neisseria gonorrhoeae, Legionella pneumophila, and Pseudomonas aeruginosa, which are associated with infections like food poisoning, sexually transmitted diseases, pneumonia, and others.

1. The Gram Negative
Bacteria

2. The Gram Stain
 Christian Gram (1884) developed gram staining
process.
 Stains interact differently with a particular type of
cell wall.
 Cells will be either Gram positive / Gram negative.

3. What is a gram (-) bacteria?
• Gram-negative bacteria are a group
of bacteria that do not retain the crystal
violet stain used in the Gram staining method of
bacterial differentiation, making positive
identification possible.

4. Gram negative bacteria
• Gram negative: A group of bacteria that do not retain the crystal
violet dye after the differential staining procedure known as Gram
staining. They appear pink due to the counterstain.
• Gram positive appears purple.
• The difference between Gram negative and Gram positive bacteria
is the cell wall structure, which accounts for the different staining
characteristics

5. Gram negative bacteria

6. Characteristics of gram negative bacteria:
1. Cell membrane (cytoplasmic).
2. Thin peptidoglycan layer (which is much thicker in gram-
positive bacteria)
3. Outer membrane containing lipopolysaccharide (LPS, which
consists of lipid A, core polysaccharide, and O antigen) in its outer
leaflet and phospholipids in the inner leaflet
4. Porins exist in the outer membrane, which act like pores for
particular molecules
5. There is a space between the peptidoglycan layer and the
secondary cell membrane called the periplasmic space

7. Characteristics of gram negative bacteria:
6. The S-layer is directly attached to the outer membrane rather
than the peptidoglycan
7. If present, flagella have four supporting rings instead of two
8. No teichoic acids or lipoteichoic acids are present
9. Lipoproteins are attached to the polysaccharide backbone.

8. Gram Negative
The Gram-negative cell wall is composed of a thin, inner layer of peptidoglycan and an
outer membrane consisting of molecules of phospholipids, lipopolysaccharides (LPS),
lipoproteins and sutface proteins. The lipopolysaccharide consists of lipid A and O
polysaccharide.

9. Characteristics of gram negative bacteria:
Thin peptidoglycan layer
(which is much thicker
in gram-positive
bacteria)

10. Characteristics of gram negative bacteria:
Outer membrane containing lipopolysaccharide (LPS, which
consists of lipid A, core polysaccharide, and O antigen) in its
outer leaflet and phospholipids in the inner leaflet

11. Characteristics of gram negative bacteria:
Porins exist in the outer membrane, which act
like pores for particular molecules

12. Characteristics of gram negative bacteria:
There is a space
between the
peptidoglycan layer
and the secondary
cell membrane called
the periplasmic
space

13. Characteristics of gram negative bacteria:
The S-layer is
directly attached to
the outer membrane
rather than the
peptidoglycan
S Layer – Crystalline layer on top of either gram-positive or gram-negative bugs. The S
Layer may increase virulence, and it may also help in adhesion.

14. Characteristics of gram negative bacteria:
If present, flagella have four
supporting rings instead of
two

15. Characteristics of gram negative bacteria:
No teichoic acids or
lipoteichoic acids are present

16. Characteristics of gram negative bacteria:
Lipoproteins are attached to the polysaccharide backbone.

17. List of Gram Negative
Bacteria

18. Acetic acid bacteria
 derive their energy from the
oxidation of ethanol to acetic
acid during fermentation.
 aerobic, rod-shaped bacteria.
 Acetic acid bacteria are airborne
and are ubiquitous in nature.

19. Acinetobacter baumannii
 It can be an opportunistic
pathogen in humans, affecting
people with compromised
immune systems
 Colloquially, A. baumannii is
referred to as 'Iraqibacter' due
to its seemingly sudden
emergence in military
treatment facilities during
the IraqWar.

20. Agrobacterium tumefaciens
 s the causal agent of crown
gall disease (the formation
of tumours)in over 140 species
of eudicots.
 Economically, A. tumefaciens is a
serious pathogen of walnuts, grape
vines, stone fruits, nut trees, sugar
beets, horse radish, andrhubarb.

21. Arcobacter
 It shows an unusually wide range of
habitats, and some species can be
human and animal pathogens
 Symptoms of infections include
diarrhea associated with abdominal
pain, nausea, and vomiting or fever

22. Arcobacter
 It shows an unusually wide range of
habitats, and some species can be
human and animal pathogens
 Symptoms of infections include
diarrhea associated with abdominal
pain, nausea, and vomiting or fever

23. Bacteroides
 obligate anaerobic bacteria
 Some species (B. fragilis, for
example) are opportunistic human
pathogens, causing infections of the
peritoneal cavity, gastrointestinal
surgery, and appendicitis via abscess
formation

24. Bdellovibrio
 obligate anaerobic bacteria
 The Bdellovibrio cell
uses hydrolytic enzymes to break
down the host cell molecules, which
it uses to elongate and form a
filament and gets the host cell
nutrients

25. Diseases that are caused by
gram negative bacteria

26. Gram Negative Spiral Bacteria
 Slender and flexible, come in a
lot of different shapes
 More rigid than spirochetes
 Ex. – Campylobacter jejuni
 Symptom – tenesmus: the
sensation of desire to defecate,
which is common and occurs
frequently , with out the
production of significant amounts
of feces (often small amounts of
mucous or blood are alone
passed).

27. Gram Negative Spirochetes
 pathogenic
 very flexible
 tightly coiled, helically coiled
 Example
 syphilis
Treponema pallidum

28. Gram Negative Spirochetes
 Most of pathogenic
 Very flexible
 Tightly coiled, helically coiled
 Example
 Lyme disease
Borrelia burgdorferi
(organism gets lodged in
tissues)

29. Gram Negative Aerobic Rods
 Legionella pneumophila
 Lower respiratory tract
infection
 Needs oxygen

30. Gram Negative Aerobic Rods
Bordetella pertussis –
whooping cough
 Needs oxygen

31. Gram Negative Aerobic Rods
 Pseudomonas aeruginosa
(pigmented)
 Needs moisture
 Common in hospitals
 Opportunistic pathogen –
causes UTI, skin, and lung
infection

32. Gram Negative Facultative Rods
Vibrio
 V. cholerae
 Most well known of group
 Very severe dysentery. Can lose
10-15 liters of water/day. Leads
to hypovolemia – low water, hardly
any water in body
 V. vulnificus
 Very pathogenic
 Can cause flesh eating disease,
if it gets in a wound
 V. parahaemolyticus
 Found in shellfish – oysters
 Halophile – loves salt (will find in
oceans, estuaries)
 Self limiting

33. Gram Negative Facultative Rods
 Enteric
 Salmonella
 Shigella
 E. coli (0157H7)

34. Gram Negative Anaerobic Rods
 Fusobacterium
 Live in between teeth
and gums
 Cause tooth abscesses
and periodontal disease
 Teeth have nothing to
anchor – bone is
destroyed

35. Gram Negative Cocci or Coccobaccilli
(plump rods)
Neisseria gonorrhoeae –
 Usually a diplococcus in
PMN
 Sexually Transmitted
Disease
 very antibiotic resistant

36. Gram Negative Cocci or Coccobaccilli
(plump rods)
 Neisseria meningitidis
very infectious and
communicable.

37. Gram Negative Cocci or Coccobaccilli (plump
rods)
 Acinetobacter baumanni
- opportunistic, UTI, skin,
and upper respiratory

38. Chlamydia Gram Negative Rods
(Transitional)
 Very short little rods
 Transitional – doesn’t hold stain well
 Do not have the ability to synthesize
own ATP, therefore and obligate
intracellular parasite of other
animals (humans)
 Can go asymptomatic for a long time
 Ex.
 C. trachomatis – STD, causes eye
infection
 C. psittaci – parrot (associated with
birds)

39. Rickettsia Gram Negative Rod
(Transitional)
 Small gram negative rods
 Transitional – doesn’t hold
stain well
 Can’t synthesize it’s own NAD,
coenzyme A, therefore an
obligate intracellular parasite
 Causative agent of Rocky
Mountain Spotted Fever
 Example
 R. Prowazekii

40. END.