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How to prepare 0.5 McFarland turbidity standard ?

0.5 McFarland turbidity standard is used to standardize the inoculum for performing antimicrobial susceptibility test. Before antibiotic disks are placed on Muller Hinton agar plate, the bacterial suspension should be matched with the turbidity of 0.5 McFarland turbidity standard. It contains 1.5 X 108  CFU/ml of bacteria. Note: If we skip matching the turbidity of […]

Rapid Plasma Reagin (RPR) test: Principle, Procedure, Result and Interpretation

INTRODUCTION : Rapid plasma reagin is commonly abbreviated as RPR. RPR test is a macroscopic, nontreponemal, flocculation test. It is a non-specific screening test for diagnosis of syphilis. Syphilis is a venereal disease caused by the spirochete Treponema pallidum.  After acquiring infection, the host forms antibodies to Treponema pallidum, in addition, the host also forms […]

Virulence factors of Pseudomonas aeruginosa

Alginate: It is a capsular polysaccharide that allows the infecting bacteria to adhere to lung epithelial cell surface and form biofilms which protects the bacteria from antibiotics and body’s immune system. Pilli: It is a surface appendage that allow adherence of the bacteria to GM -1 ganglioside receptor on host epithelial surface Neuraminidase: It removes […]

D test- A test for detection of inducible Clindamycin resistance in Staphylococcus aureus

D-test  detects inducible clindamycin resistance in Staphylococcus aureus by double disk diffusion using Clindamycin(2ug) and Erythromycin (15 ug) disks. Erythromycin is a macrolide and clindamycin is a lincosamide, two distinct classes of antimicrobial agents that inhibit protein synthesis by binding to the 50S ribosomal subunits of bacterial cells. Resistance in Staphylococcus can occur to both of these antimicrobial […]

ONPG (o-Nitrophenyl-β-D-Galactopyranoside) test: Principle, Procedure, Result and Interpretation

Principle: o-Nitrophenyl-β-D-Galactopyranoside (ONPG) is structurally similar to lactose except that o-nitrophenol has been substituted for glucose. On hydrolysis through the action of enzyme β-galactosidase, ONPG which is a colorless compound cleaves into two residues, galactose and o-nitrophenol which is yellow in color. Development yellow color provides visual evidence of hydrolysis of ONPG. Views – 10221

Superoxol Test: Rapid presumptive test for Neisseria gonorrhoea (Gonococcus)

Superoxol test is a rapid test that can be used for presumptive identification of Neisseria gonorrhoea. It is similar to Catalase test in which 3% hydrogen peroxide (H2O2) solution is used but 30% hydrogen peroxide is used in this test. N. gonorrhoeae strains produce immediate, vigorous bubbling when the colony is emulsified with 30% H2O2. […]

Acetamide Utilization Test: Principle, Procedure, Expected results, Quality Control and Limitation

Acetamide Utilization test: This test is useful in differentiation of nonfermentative gram-negative bacteria, particularly Pseudomonas aeruginosa. Principle of Acetamide Utilization Test Acetamide utilization test is used to check the ability of an organism to use acetamide as a sole source of carbon. Organisms that can grow on this medium are able to deaminate acetamide and release […]

X and V factor test: Identification test for Haemophilus

Members of the genus Haemophilus are fastidious in nature. i.e. they require extra nutritional supplements in order to grow.  Some Hemophilus require only X factor (haemin-which is present on blood agar)  or only V factor (Nicotinamide-adenine dinucleotide, NAD- which is present within the blood cells) or a combination of both. So,  Haemophilus influenzae grows only […]

Definition of Staphylococci

Members of the genus Staphylococcus are gram positive cocci of size 0.5-1.5 um in diameter. They occur in singles, pairs, tetrads, short chains and irregular grape like cluster. They are nonmotile and nonspore- forming. Most species are catalase positive and are facultative anaerobes, grows better under aerobic than anaerobic conditions. The anaerobic species Staphylococcus saccharolyticus […]

Penicillin: Types, Mechanism of action, Pharmacology, Spectrum of Activity and Adverse effects

Penicillin consists of a group of natural and semisynthetic antibiotics containing the chemical nucleus, 6-aminopenicillanic acid, which consists of a β-lactam ring fused to a thiazolidine ring. The naturally occurring compounds are produced by fungi of genus Penicillium. The penicillins differ from one another in the substitution at position 6, where changes in the side […]

Definition of Enterococcus

Members of the genus Enterococcus are gram positive cocci that occur in single or arranged in pairs or short chains. Cells may appear coccobacillary when gram stains are prepared from growth on solid media. Cells tend to be ovoid and in chains when gram stains are prepared form growth in thioglycolate broth. They are facultative […]

Importance of sputum gram stain: Acceptance and Rejection criteria

Gram stain of sputum is a valuable test for the requesting physician. It helps the physician to judge whether the patient he/she has counseled to obtain the sputum has really submitted the sputum sample or merely an oropharyngeal secretion i.e. saliva. Generally, while preparing the sputum smear, purulent part of the sputum is taken and […]

Cystine Lactose Electrolyte Deficient (CLED) agar

Cystine Lactose Electrolyte Deficient (CLED) agar is a type of differential medium generally used for isolation and enumeration of bacteria from urine specimen form patients suspected of urinary tract infection.  CLED agar supports the growth of all potential uropathogens such as gram negative bacilli belonging to Enterobacteriaceae and gram positive cocci such as Enterococcus spp, […]

Descriptions of Primary, Secondary and Special Skin lesions

Primary Lesions Primary lesions include the following: Macules are up to 1 cm and are circumscribed, flat discolorations of the skin. Examples: freckles, flat nevi. Patches are larger than 1 cm and are circumscribed, flat discolorations of the skin. Examples: vitiligo, senile freckles, measles rash. Papules are up to 1 cm and are circumscribed, elevated, […]

Normal flora of skin

Skin provides good examples of various microenvironments. Skin can be compared to geographic regions of earth: the desert of the forearm, the cool woods of the scalp, and the tropical forest of the armpit. The composition of the dermal microflora varies from site to site according to the character of the microenvironment. A different bacterial […]

Biological Safety Cabinets: types and use

Biological safety cabinets (BSCs) are a form of engineering control that is used throughout the microbiology laboratory. These hoods are a type of containment barrier that protects the laboratory worker from the aerosolized transmission of organisms. Any procedure that involves the risk of generating aerosols should be handled in a BSC. Samples sent to microbiology […]

Specimen collection, transport and methods for specimen collection for diagnosis of anaerobes

Proper collection of specimens is of utmost importance to avoid contamination of the collected specimen with normal microbial flora and prompt transport to the laboratory for processing is very essential for the diagnosis of infectious diseases. Isolation of anaerobes from clinical specimens, determination of the numbers of anaerobes in the specimen, and establishing the clinical […]

Characteristics that a microbe must possess to cause infection in human

In order to cause infection, a successful pathogen must be able to: Enter and establish in the human host Acquire nutrients from the host Avoid or circumvent the host’s innate defenses and a powerful immune system Multiply in numbers Disseminate to a preferred site, if necessary and eventually Be transmitted to a new susceptible host […]

Optochin Sensitivity Test: Principle, Procedure and Interpretation

Principle: Optochin (ethyl hydrocupreine hydrochloride) sensitivity test is used for the presumptive identification of alpha-hemolytic streptococci as Streptococcus pneumoniae. Optochin is used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci.  Streptococcus pneumoniae strains are sensitive to Optochin although some strains are optochin-resistant and other alpha-hemolytic streptococcal species are Optochin-resistant. The Optochin test is performed on […]

Introduction to Human Papiloma VIrus

Human papillomavirus (HPV) is a virus that infects the skin, genital area and lining of the cervix. There are about 100 different types of papilloma viruses (Some types of papillomaviruses cause warts on the skin, some types cause warts in the anal and genital areas, and some types cause cervical cancer.) Many different HPV types […]

Infections caused by anaerobes and features of anaerobic infections

Infections caused by anaerobic bacteria occur in all parts of body. These infected tissues usually contain a mixture of several kinds of anaerobes and frequently also contain aerobic and facultative bacteria. The types of infection commonly produced by anaerobic bacteria are as follows:  Intra-abdominal infections: Abscesses, post operative wound infections and generalized peritonitis produced by […]

Anaerobes: Why can’t anaerobic organism survive in presence of oxygen?

Anaerobes or anaerobic organisms are those that cannot grow in presence of oxygen. Oxygen is toxic for them so they must rely on other substances as terminal electron acceptor. Their metabolism is of fermentative type in which they reduce available organic compounds to various end products like organic acids and alcohols. The main reason behind […]