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Sub-Topic: Bacterial Shape, Size, and Arrangement (Morphology)
Part 1: Bacterial Size
irst, let's appreciate how small these organisms are. Bacteria are measured inmicrometers
F
(µm).
1 micrometer (µm) = 1/1,000,000 of a meter.
●
● Most medically important bacteria are about0.2 to2.0 µmin diameter or width and1.0
to 10.0 µmin length.
● To give you perspective, a single red blood cell is about 7 µm in diameter. So you could
line up several bacteria across one of your own blood cells!
Part 2: Bacterial Shapes (Morphology)
Bacteria are classified based on their basic shape. Think of these as the main families.
1. C occi (singular: Coccus)
○ Meaning:From the Greek wordkokkos, meaning "berry."
○ Description:These are spherical or oval-shaped bacteria.
2. Bacilli (singular: Bacillus)
○ Meaning:From the Latin wordbaculus, meaning "rod"or "stick."
○ Description:These are rod-shaped bacteria.
3. Spiral Forms
○ Description:These are bacteria that are curved orhelical. This group has a few
important sub-types that you must know.
■ Vibrio:A curved or comma-shaped rod. It has lessthan one complete twist.
■ Spirillum:A rigid, helical bacterium with two ormore curves. It moves using
external flagella.
■ Spirochete:A flexible, helical bacterium that lookslike a corkscrew. It has a
unique internal flagella system for movement.
Part 3: Bacterial Arrangements
his is where it gets really useful. How bacteria group themselves after they divide gives us
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major clues to their identity. The arrangement is determined by theplane of cell division.
A. Arrangements of Cocci
Imagine a single spherical bacterium dividing.
● I f it divides in one plane:It can form pairs or chains.
○ Diplococci (Pairs):The cocci remain in pairs afterdivision.
■ Key Medical Examples:
■ Neisseria gonorrhoeae(causes gonorrhea). They aretypically kidney-bean
shaped.
■ Streptococcus pneumoniae(a major cause of pneumoniaand meningitis).
They are lancet-shaped or flame-shaped.
○ Streptococci (Chains):The cocci divide repeatedlyin one plane and remain
attached, forming a chain.
■ Analogy:Think of aStrapor aStringof pearls.
■ Key Medical Example:Streptococcus pyogenes(causesstrep throat and
rheumatic fever).
● If it divides in two planes at right angles:It formsgroups of four.
○ Tetrads (Groups of 4):
■ Key Medical Example:Micrococcusspecies.(Often non-pathogenic,found on
skin).
● If it divides in three planes at right angles (like cutting a cube):It forms cubical
packets of eight.
○ Sarcinae (Packets of 8):
■ Key Medical Example:Sarcina ventriculi.
● If it divides in multiple planes, randomly:It formsgrape-like clusters.
○ Staphylococci (Clusters):
■ Analogy:Think of aStaffof grapes.
■ Key Medical Example:Staphylococcus aureus(causesboils, abscesses, and
toxic shock syndrome).
linical Point:This is incredibly useful in the lab.If a doctor gets a report of
C
"Gram-positive cocci in clusters" from an abscess, they immediately suspect
Staphylococcus aureusand can start appropriate treatmenteven before the final
identification is ready. If the report says "Gram-positive cocci in chains" from a
throat swab, they'll think ofStreptococcus pyogenes.
B. Arrangements of Bacilli
ods generally divide only across their short axis (a process called transverse fission), so they
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have fewer arrangements than cocci.
● C occobacilli:These are short, plump rods that looklike a mix between a coccus and a
bacillus.
○ Key Medical Example:Haemophilus influenzae,Brucellaspp.
● Diplobacilli (Pairs):Two rods paired together.
● Streptobacilli (Chains):Rods arranged in chains.
● P
alisades (Fence-like):The rods line up side-by-side, like a picket fence. This is also
called a "coryneform" arrangement.
○ Key Medical Example:Corynebacterium diphtheriae(causesdiphtheria). The
bacteria look like "Chinese letters" or a V-shape under the microscope because they
snap back on each other after division. This is a classic exam description.
C. Pleomorphism
ome bacteria don't have a single, fixed shape. They can vary in shape and size even within a
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single culture. This property is calledpleomorphism.
● A
classic example isMycoplasma, which has no cellwall at all, so it's completely
pleomorphic and can look like anything.
Summary Table: Bacterial Morphology & Arrangement
(This table blends information from Sastry and Murray for your high-yield review)
Shape Arrangement Description ey Medical
K xam Nugget
E
Example / Clinical
Pearl
Cocci iplococci
D Cocci in pairs. treptococcus
S . pneumoniae
S
(Pairs) pneumoniae, are
Neisseriaspp. lancet-shaped.
Neisseriaare
kidney-bean
shaped and
are
Gram-negative
.
treptococci
S occi in chains
C treptococcus
S eeing chains
S
(Chains) of varying pyogenes of
lengths. (Group A Gram-positive
Strep) cocci is highly
suggestive of a
Streptococcal
infection.
taphylococci
S occi in
C taphylococcu
S eeing clusters
S
(Clusters) irregular, s aureus of
rape-like
g ram-positive
G
clusters. cocci is highly
suggestive of a
Staphylococca
l infection
(e.g., abscess).
Tetrads occi in
C icrococcus
M ften
O
square groups spp. considered a
of four. contaminant
from the skin
in clinical
samples.
Sarcinae occi in
C Sarcinaspp. ot commonly
N
cubical pathogenic.
packets of
eight.
Bacilli Coccobacilli ery short,
V aemophilus
H . influenzaeis
H
plump rods. influenzae, a major cause
Brucella of meningitis
and
pneumonia,
especially in
children.
alisades
P ods arranged
R orynebacteri
C lassic
C
(Coryneform) side-by-side um diphtheriae description is
or at sharp "Chinese letter
angles. pattern" or
"club-shaped."
(High-yield
for
INICET/NEET
PG)
Spiral Vibrio urved,
C Vibrio cholerae he cause of
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comma-shape cholera, a
d rod. severe
diarrheal
disease.
Spirillum igid, helical
R pirillum minus
S ot as
N
shape with (causes a type clinically
external of rat-bite common as
flagella. fever) the others.
Spirochete lexible,
F reponema
T heir unique
T
corkscrew pallidum endoflagella
shape with (Syphilis), allows them to
internal Borrelia(Lyme move in a
flagella. Disease) corkscrew
motion,
enabling them
to drill through
viscous tissue.
(High-yield
concept)
For Your Exams
Practical Exam (OSPE/Viva)
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● potter:A Gram stain slide is shown.
● Examiner:"Describe the morphology of the bacteriayou see on this slide."
● Your Ideal Answer:"Sir/Ma'am, on this slide, I cansee[Color, e.g., purple]colored
organisms, indicating they are[Gram-positive/negative].Their shape is
[spherical/rod-shaped], and they are arranged in[chains/clusters/pairs].Therefore,
the morphology is Gram-[positive/negative] [cocci/bacilli]in
[chains/clusters/pairs]."
○ Example: "Gram-positive cocci in clusters."
Entrance Exams (MCQs)
CQ 1 (NEET PG Style): A Gram stain from a skin abscess reveals purple, spherical bacteria
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arranged in irregular clusters. Which of the following organisms is the most likely cause?
A. Streptococcus pyogenes
B. Neisseria meningitidis
C. Staphylococcus aureus
D. Corynebacterium diphtheriae
Answer: C. (The keywords are purple [Gram-positive], spherical [cocci], and clusters. This
classic description points directly to Staphylococcus).
CQ 2 (INICET Style): A microbiologist observes a bacterium with a flexible, helical shape that
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exhibits a corkscrew-like motility. This motility is most likely due to the presence of:
A. Peritrichous flagella
B. A single polar flagellum
C. Axial filaments (Endoflagella)
D. Pili
Answer: C. (The description of a flexible, helical bacterium with corkscrew motility is classic
for a spirochete. Their unique motility is provided by their internal flagella, known as axial
filaments or endoflagella).
