Microbial Diagnostic Disks: Introduction, List of Disks, Uses, and

Introduction of Microbial Diagnostic Disks

Table of Contents

Microbial Diagnostic Disks make it easy to presumptive identification of organisms. They are simple and easy to handle for the identification of bacteria in the Clinical Microbiology Laboratory. Some common examples of such disks are optochin, oxidase, and bacitracin.

List of Microbial Diagnostic Disks

Novobiocin
Cefoxitin
Optochin
Oxidase disks
Bacitracin 10 U
Bacitracin 0.04 U
ONPG disks
X, V, and XV disks

Uses of Microbial Diagnostic Disks

Novobiocin: Staphylococcus saprophyticus is an intrinsic resistance. This disc uses to differentiate S. saprophyticus from other coagulase-negative staphylococci by the overnight incubation disc test method.
Optochin: It uses for the presumptive identification of Streptococcus pneumoniae, which is optochin-sensitive, from other alpha-hemolytic streptococci such as viridans streptococci, which are resistant.
Oxidase disks: To differentiate oxidase-positive organisms like Pseudomonas, Aeromonas, Vibrio, Haemophilus, Neisseria, Brucella, etc.
Bacitracin (0.04 U): It uses to differentiate Streptococcus pyogenes from other ß hemolytic streptococci, Streptococcus agalactiae . S. pyogenes is sensitive whereas S. agalcatiae is resistant.
Bacitracin 10 U: It is applied in a sputum culture plate (chocolate agar) to inhibit normal flora, including gram-positive bacteria, such as streptococci, and most species of Neisseria whereas to support the growth of Haemophilus which is resistant.
ONPG disks: The ONPG test is useful in differentiating members of the family Enterobacteriaceae and genera Neisseria based on b-D-galactosidase activity. ONPG stands for o-Nitrophenyl-b-D-galactopyranoside. Positive –Escherichia coli ATCC 25922 and Neisseria lactamica ATCC 23971 whereas negative Proteus mirabilis ATCC 12453 and Neisseria gonorrhoeae ATCC 43069.
X, V, and XV disks: For Haemophilus and its species identification.
Cefoxitin disk: It is used for screening Methicillin-Resistant Staphylococcal aureus (MRSA) and the Zone of inhibition (ZOI) should be less than 21 mm.

Haemophilus species Identification Using X, V, and XV Growth Factors Disks

Isolation of numerous Haemophilus species on the basis of growth characteristics are as follows-

Species	X Disk	V disk	CO₂	Hemolysis on horse blood agar
Haemophilus influenzae	+	+	–	–
H. parainfluenzae	–	+	–	–
H. ducreyi	+	–	variable	variable
H. aegyptius	+	+	–	–
H.haemolyticus	+	+	–	+
H. parahaemolyticus	–	+	–	+
H. aphrophilus	+	–	+	–
H. paraaphrophilus	–	+	+	–

Table: Haemophilus species Identification Using X, V, and XV disks

Table: Haemophilus species Identification Using X, V, and XV Growth Factors Disks

Keynotes

Haemophilus influenzae is xylose positive while Haemophilus aegyptius is negative.
The x-factor is hemin and it is required for the synthesis of respiratory enzymes and is also called a heat-stable growth-promoting substance present in RBCs.
V factor is nicotinamide-adenine dinucleotides (NAD)and it is applied as an electron carrier and is also known as a heat-labile vitamin-like substance present in red blood cells.
Chocolate agar is a very useful medium to isolate fastidious organisms in Microbiology Laboratory from various clinical specimens like sputum ( H. influenzae), urethral discharge ( N. gonorrhoeae), CSF/blood (N. menigitidis).
Nutrient agar is used for X, V, and XV factors assaying for Haemophilis species identification.