[Microbiology] Atlas of Growth Patterns on Agar

Growth Patterns on Agar, Bacterial Growth, Atlas of Growth Patterns on Agar, A Photographic Atlas for the Microbiology Laboratory

Purpose

Recognizing different bacterial growth morphologies on agar plates is a useful and often crucial step in the identification process. Agar slants are typically used for cultivation of pure cultures. Bacteria also frequently display distinct morphological color and texture on agar slants.

Principle

When a single bacterial cell is deposited on a solid nutrient medium, it begins to divide. One cell makes two, two make four, four make eight . . . one million make two million, and so on. Eventually a colony appears where the original cell was deposited. Once the purity of a colony has been confirmed by an appropriate staining procedure (Sections 5 and 6), cells can then be transferred to a sterile medium to begin a pure culture. Color, size, shape, and texture of microbial growth are determined by the genetic makeup of the organism. However, organismal genetic expression is also greatly influenced by environmental factors including nutrient availability, temperature, and incubation time. Colony characteristics may be viewed with the naked eye or with the assistance of a colony counter (Figure 3-1). The basic categories of growth include colony shape, margin (edge), elevation, color, and texture (Figure 3-2). Colony shape may be described as circular, irregular, or punctiform (tiny). The margin may be entire (smooth, with no irregularities), undulate (wavy), lobate (lobed), filamentous, or rhizoid (branched like roots). Colony elevations include flat, raised, convex, pulvinate (very convex), and umbonate (raised in the center). Colony texture may be moist, mucoid, or dry. Pigment production is another useful characteristic and may be combined with optical properties such as opaque, translucent, shiny, or dull.

3-1 THE COLONY COUNTER Subtle differences in colony
shape and size can best be viewed on the colony counter.
The transmitted light and magnifying glass allow observation 

of greater detail, however, colony color is best 

determined with reflected light. The grid in the background is used
as a counting aid. Each big square is a square centimeter.  

3-2 A SAMPLING OF BACTERIAL COLONY FEATURES These terms
are used to describe colonial morphology. Descriptions also should
include color, surface characteristics (dull or shiny), consistency (dry,
butyrous-buttery, or moist) and optical properties (opaque or translucent).  

Figures 3-3 through 3-31 show a variety of bacterial colony forms and characteristics. Where applicable, contrasting environmental factors are indicated. Figures 3-32 and 3-34 show growth characteristics on agar slants.  

3-3 ENTEROCOCCUS FAECIUM GROWN ON NUTRIENT AGAR The
colonies are white, circular, convex, smooth, and have an entire margin.E. faecium (formerly known as Streptococcus faecium) is found in human
and animal feces.  

3-4 STAPHYLOCOCCUS EPIDERMIDIS GROWN ON SHEEP BLOODAGAR The colonies are white, raised, circular, and entire. S. epidermidisis an opportunistic pathogen.  

3-5 MICROCOCCUS LUTEUS GROWN ON BRAIN HEART INFUSIONAGAR These colonies are yellow, smooth, and convex with a regular
margin. They range in size from 1 to 3 mm. M. luteus is common in soil,
dust, and on human skin.  

3-6 KOCURIA ROSEA GROWN ON BRAIN HEART INFUSION AGARPink, punctiform (these are less than 1 mm in diameter), smooth, regular 

colonies typify K. rosea, an inhabitant of water, dust, and salty foods.  

3-7 SARCINA AURANTIACA GROWN ON BRAIN HEART INFUSIONAGAR S. aurantiaca produced yellow-orange, smooth, convex, regular
colonies on BHIA. These are between 1 to 3 mm in diameter  

3-8 RHODOCOCCUS RHODOCHROUS GROWN ON BRAIN HEART INFUSIONAGAR These colonies are reddish-pink, smooth, and convex with a regular
margin. They are about 1 mm in diameter. Rhodococcus species are soil organisms.  

3-9 COMPARISON OF FOUR BACILLUS SPECIES A B. cereus grown on Blood Agar produces distinctively large (up to 7 mm), gray, granular, irregular colonies. They often produce a “mousy” smell. Also note the extensions of growth along the streak line. B B. anthracis colonies resemble B. cereus,
but are usually smaller and adhere to the medium more tenaciously. C B. mycoides produces rapidly spreading, rhizoid colonies. D This unknown Bacillus isolated as a laboratory contaminant produced wrinkled, irregular colonies with an undulate (wavy) margin.  

3-10 “MEDUSA HEAD” COLONIES OF CLOSTRIDIUM

SPOROGENES ON BLOOD AGAR These irregularly circular

colonies have a raised, yellow center and a flat, spreading

edge of tangled filaments (reminiscent of the mythological

creature Medusa, who had snakes for hair!). They vary in

size from 2 to 6 mm.

3-11 PROVIDENCIA STUARTII GROWN ON NUTRIENT AGAR The

colonies are shiny, buff, and convex. P. stuartii is a frequent isolate in

urine samples obtained from hospitalized and catheterized patients.

P. stuartii is highly resistant to antibiotics  

3-12 KLEBSIELLA PNEUMONIAE GROWN ON NUTRIENT AGAR The

colonies are mucoid, raised, and shiny. While it is a normal inhabitant

of the human intestinal tract, it is associated with community-acquired

pneumonia and nososomial urinary tract infections.  

3-13 ALCALIGENES FAECALIS COLONIES ON SHEEP BLOOD AGAR 

The colonies of this opportunistic pathogen are umbonate with an opaque

center and a spreading edge. A Side view. Note the raised center. 

B Close-up of the A. faecalis colonies showing spreading edge.

3-14 CITROBACTER KOSERI GROWN ON SHEEP BLOOD AGAR The

colonies are round, smooth, and opaque with a regular margin. This

species is also able to partially hemolyze red blood cells (-hemolytic),

as evidenced by the greening around each colony. They range in size

from 1 to 2 mm

3-15 ERWINIA AMYLOVORA GROWN ON BRAIN HEART INFUSION

AGAR These colonies are whitish, transluscent, spreading, and umbonate. 

Erwinia species are plant pathogens

3-16 RHIZOBIUM LEGUMINOSARUM GROWN ON BRAIN HEART

INFUSION AGAR The colonies are convex, circular, and filamentous.

They are translucent at the edges and about 5 mm in diameter.

R. leguminosarum is capable of producing root nodules (tumors)

in many legumes and subsequently fixing atmospheric nitrogen.  

3-17 DEINOCOCCUS RADIODURANS GROWN ON TRYTPICASE SOY

AGAR These small (between 1 and 2 mm in diameter), round, convex,

and regular colonies took 36 hours to develop the orange color. This

species is highly resistant to ionizing radiation.  

3-18 MYCOBACTERIUM SMEGMATIS GROWN

ON SHEEP BLOOD AGAR The colonies of this

slow growing relative of M. tuberculosis are

punctiform.  

3-19 CORYNEBACTERIUM XEROSIS GROWN ON SHEEP BLOOD

AGAR A As seen in this view from the side, the colonies are dull, buff,

and convex. B Close-up of circular C. xerosis colonies. C. xerosis is rarely

an opportunistic pathogen.

  

3-20 UMBONATE COLONY OF AN ANAEROBIC LAB CONTAMINANT

The colony on the left is truly umbonate. The one on the right is getting

there. Their diameters are about 3 mm.

3-21 STREPTOMYCES GRISEUS GROWN ON BRAIN HEART INFUSION

AGAR These colonies are circular and ridged with a granular appearance. 

At a later stage of development, they produce yellow reproductive

spores. Growth of streptomycetes is associated with an “earthy” smell.

This one plate fragranced the entire incubator!  

3-22 SWARMING GROWTH PATTERN Members of the genus

Proteus will swarm at certain intervals and produce a pattern of

concentric rings due to their motility. This is a photograph of

P. vulgaris demonstrating swarming behavior on DNase agar.  

3-23 MUCOID COLONIES Pseudomonas aeruginosa grown on Endo

agar demonstrates a mucoid texture. P. aeruginosa is found in soil and

water, and can cause infections in burn patients.  

3-24 BUTYROUS COLONY OF AN UNKNOWN SOIL ISOLATE This

12 mm colony was found on a Glycerol Yeast Extract plate inoculated

with a diluted soil sample. It was almost liquid in composition, something 

that is indicated by its contact with the yellow colony to its right.  

3-25 CHROMOBACTERIUM VIOLACEUM GROWN ON SHEEP BLOOD

AGAR C. violaceum produces shiny, purple, convex colonies. It is found

in soil and water, and rarely produces infections in humans.  

3-26 INFLUENCE OF NUTRIENT AVAILABILITY ON PIGMENT PRODUCTION

Pigment production may be influenced by environmental factors such as

nutrient availability. Chromobacterium violaceum produces a much more intense

purple pigment when grown on Trypticase Soy Agar (left) than when grown on

Nutrient Agar, a less nutritious medium (right).  

3-27 INFLUENCE OF AGE ON PIGMENT PRODUCTION A Serratia marcescens grown on Sheep Blood Agar after 24 hours. B The same plate of S. marcescens after 48 hours. Note in particular the change in the 3 colonies in the lower right (encircled)  

3-28 EFFECT OF AGE ON COLONY MORPHOLOGY 

A Close-up of Bacillus subtilis on Sheep Blood Agar after 24 hours of incubation. 

B Close-up of B. subtilis on Sheep Blood Agar after 48 hours. Note the wormlike appearance.  

3-29 DIFFUSIBLE PIGMENT OF PSEUDOMONAS AERUGINOSA 

The blue-green pigment pyocyanin often makes P. aeruginosa easy to identify.  

3-30 TWO MIXED SOIL CULTURES ON NUTRIENT AGAR 

These plates show the morphological diversity present in two soil samples.  

3-31 THREE THROAT CULTURES ON SHEEP BLOOD AGAR

A There are probably five different species on this plate. 

B Note the-hemolysis (darkening of the agar; see page 61 for more information) shown by much of the growth. C This is a close-up of the same plate as in B. Note the weak Beta-hemolysis of the

white colony in the upper right (arrow). White growth with Beta-hemolysis is characteristic of Staphylococcus aureus.

3-32 PIGMENT PRODUCTION ON SLANTS 

From left to right, Staphylococcus epidermidis (white), Pseudomonas aeruginosa (green),

Chromobacterium violaceum (violet), Serratia marcescens (red/orange),

Kocuria rosea (rose), and Micrococcus luteus (yellow)  

3-33 INFLUENCE OF TEMPERATURE ON PIGMENT PRODUCTION 

Serratia marcescens was grown for 48 hours on Trypticase Soy Agar slants at five
different temperatures. From left to right: 25°C, 30°C, 33°C, 35°C, and 37°C.
A difference of 2°C makes the difference between being pigmented or not!  

3-34 GROWTH TEXTURE ON SLANTS 

From left to right, Bacillus spp. (flat, dry), Alcaligenes

faecalis (spreading edge), Mycobacterium phlei

(crusty/friable), Lactobacillus plantarum (transparent, barely visible).  

Suggested Reading

- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011