Laboratory Diagnosis of Bacterial Disease

Colony Picker: Principle, Protocol, and Advantages 

Colony Picker: Principle, Protocol, and Advantages 

Automated colony pickers are the equipment that helps pick the desired colony from a cluster of different colonies from any culture media.

Moraxella catarrhalis: Properties, Lab Diagnosis

Moraxella catarrhalis: Properties, Lab Diagnosis

Moraxella catarrhalis is a small gram-negative diplococci that resembles Neisseria and causes otitis media in children.

Demonstration of Treponema pallidum Using Dark-Field Microscopy

Demonstration of Treponema pallidum Using Dark-Field Microscopy

Dark field microscopy is the most specific and sensitive technique to diagnose syphilis when an active chancre or condyloma lata is present.

Critical (Panic) Values in Microbiology

Critical (Panic) Values in Microbiology

Presence of bacteria in sterile body fluids, isolation of Mycobacterium tuberculosis or any other bio-terrorism agents should be reported immediately.

Rejection Criteria for Microbiological Specimens

Rejection Criteria for Microbiological Specimens

Clerical errors, inappropriate containers/conditions, inappropriate or inadequate specimens, delay in transit lead to sample rejection.

Bacteroides fragilis: Properties, Pathogenesis, Lab Diagnosis

Bacteroides fragilis: Properties, Pathogenesis, Lab Diagnosis

Bacteroides fragilis is a gram-negative, pleomorphic anaerobe responsible for variety of anaerobic infections including peritonitis.

Actinomyces: Properties, Disease, Lab Diagnosis

Actinomyces: Properties, Disease, Lab Diagnosis

Actinomyces is a heterogeneous group of gram-positive, pleomorphic bacilli responsible for the disease actinomycosis.

Francisella tularensis: Properties, Pathogenesis, Lab Diagnosis

Francisella tularensis: Properties, Pathogenesis, Lab Diagnosis

Francisella tularensis is a gram-negative coccobacillus and is the etiologic agent of tularemia. It is transmitted to humans from rodents and rabbits.

Burkholderia pseudomallei: Properties, Lab Diagnosis

Burkholderia pseudomallei: Properties, Lab Diagnosis

Burkholderia pseudomallei is a water- and soil-borne gram-negative bacterium. Causative agent of melioidosis, it is regarded as a potential bioterror agent.

Nocardia: Properties, Pathogenesis, Lab Diagnosis

Nocardia: Properties, Pathogenesis, Lab Diagnosis

Nocardia species are gram-positive, variably acid-fast, filamentous, and strictly aerobic organisms belonging to the actinomycetes group.

Legionella pneumophila: Properties, Pathogenesis, Lab Diagnosis

Legionella pneumophila: Properties, Pathogenesis, Lab Diagnosis

Legionella is a gram-negative pleomorphic rod, responsible for a serious type of pneumonia called Legionnaires' disease.

Laboratory Diagnosis of Mycobacterium tuberculosis Infection

Laboratory Diagnosis of Mycobacterium tuberculosis Infection

Ziehl-Neelsen staining, culture in Lowenstein-Jensen medium and GeneXpert MTB/RIF assay are useful methods for the diagnosis of tuberculosis.

Corynebacterium diphtheriae: Pathogenesis, Lab Diagnosis

Corynebacterium diphtheriae: Pathogenesis, Lab Diagnosis

Gram positive club shaped (chinese letter form) bacteria responsible for diphtheria.

Clostridium perfringens: Properties, Diseases, Diagnosis

Clostridium perfringens: Properties, Diseases, Diagnosis

Commensal in the large intestine of humans and is responsible for the disease, gas gangrene.

Clostridium tetani: Properties, Pathogenesis, Lab Diagnosis

Clostridium tetani: Properties, Pathogenesis, Lab Diagnosis

Gram positive, obligate anaerobe, looks like drum-stick shaped and causes tetanus.

Clostridium botulinum: Properties, Pathogenesis, Lab Diagnosis

Clostridium botulinum: Properties, Pathogenesis, Lab Diagnosis

Gram positive, anaerobic rod which produces powerful neurotoxin "botulinum toxin".

Yersinia pestis: Properties, Disease, Lab Diagnosis

Yersinia pestis: Properties, Disease, Lab Diagnosis

Yersinia pestis is a short gram-negative rod, responsible for deadly zoonotic disease, plague.

Dark-field Microscopy: Principle and Uses

Dark-field Microscopy: Principle and Uses

Dark-field microscopy is used for the observation of living, unstained cells. The specimen is brightly illuminated while the background is dark.

Bacillus anthracis: Properties, Pathogenesis, Lab Diagnosis

Bacillus anthracis: Properties, Pathogenesis, Lab Diagnosis

A potential biological weapon which causes important zoonotic disease called anthrax.

Streptococcus agalactiae (GBS): Properties, Pathogenesis, Lab Diagnosis

Streptococcus agalactiae (GBS): Properties, Pathogenesis, Lab Diagnosis

Streptococcus agalactiae (GBS), is a beta-hemolytic, CAMP test positive, gram-positive cocci. It can cause severe infection in newborns.

Bactec Automated Blood Culture System

Bactec Automated Blood Culture System

BACTEC by BD diagnostics utilizes fluorescent technology and detects the growth of organisms in the blood culture bottles.

Antigens in disease diagnosis

Antigens in disease diagnosis

Detection of specific antigens present in the sample (blood, urine, or CSF) using specific antibodies is the mainstay for the rapid diagnosis of infections.

Laboratory Diagnosis of Leptospirosis

Laboratory Diagnosis of Leptospirosis

Leptospirosis is diagnosed using dark-field microscopy, microscopic agglutination test (MAT) or using IgM ELISA.

Mycoplasma pneumoniae: Properties, Disease, Diagnosis

Mycoplasma pneumoniae: Properties, Disease, Diagnosis

This smallest free-living bacteria does not have cell wall (so pleomorphic in shape) causes walking pneumonia.

Klebsiella pneumoniae: Properties, Diseases, Lab Diagnosis

Klebsiella pneumoniae: Properties, Diseases, Lab Diagnosis

This gram-negative capsulated rod-shaped bacteria causes hospital acquired pnuemonia, UTI and blood stream infection.

Pus Sample: Collection, Processing, Staining and Culture

Pus Sample: Collection, Processing, Staining and Culture

Pus aspirate is the preferred sample. S. aureus, Strep pyogenes, and members of Enterobacteriaceae are major pathogens.

MALDI-TOF Mass Spectrometry: Principle, Applications in Microbiology

MALDI-TOF Mass Spectrometry: Principle, Applications in Microbiology

MALDI-TOF MS offers species-level identifications in minutes at low costs with accuracy that matches or even exceeds the conventional system.

Blood Culture: Indications, Timing, and Volume

Blood Culture: Indications, Timing, and Volume

The overall volume of blood cultured is more critical in increasing organism yield. More the blood volume cultured greater the chance of isolating pathogen.

Blood Culture Test: Information for the Patient

Blood Culture Test: Information for the Patient

Physicians may prefer blood culture in a number of suspected disease conditions such as enteric fever, infective endocarditis, meningitis, pneumonia, etc.

Pneumococcal Urinary Antigen Testing (UAT)

Pneumococcal Urinary Antigen Testing (UAT)

Pneumococcal urinary antigen testing is a rapid assay that detects Streptococcus pneumoniae antigen (C-polysaccharide) in the urine of patients.

Urea Breath Test (UBT) for H.Pylori

Urea Breath Test (UBT) for H.Pylori

Patients swallow 14C-urea capsule. Test urea will be broken down, if infected, and a lot of 14CO2 produced which is detected in the breathe.

Cary-Blair transport medium: Composition, Preparation, Uses

Cary-Blair transport medium: Composition, Preparation, Uses

Cary-Blair transport medium is used to transport clinical specimens suspected to contain enteric pathogens, including Shigella, Salmonella, Vibrio.

X and V factor test for Haemophilus

X and V factor test for Haemophilus

Haemophilus species require accessory growth factors such as X (hemin), V (NAD) or a combination of both.

Löwenstein–Jensen (LJ) Medium: Preparation, Uses

Löwenstein–Jensen (LJ) Medium: Preparation, Uses

M. tuberculosis gives rough, raised, dry, non-pigmented (cream/buff colored) colonies on Lowenstein Jensen (LJ) medium.

TPHA: Principle, Procedure, Results, Interpretations

TPHA: Principle, Procedure, Results, Interpretations

TPHA is a treponemal test for the serologic diagnosis of syphilis, based on the principle of passive hemagglutination. It detects IgG and IgM.

Helicobacter Pylori Tests and Results

Helicobacter Pylori Tests and Results

To diagnose H. pylori infections, urea breathe test, fecal antigen test and rapid urease test are most widely used.

GeneXpert MTB/RIF Assay

GeneXpert MTB/RIF Assay

GeneXpert MTB/RIF assay is a NAAT that simultaneously detects DNA of M.tuberculosis complex and resistance to rifampin (i.e. mutation of the rpoB gene).

Quellung reaction: Principle, Procedure, Results

Quellung reaction: Principle, Procedure, Results

Quellung reaction is the gold standard technique for serotyping S. pneumoniae. Capsule get “swollen” and is more visible.

Widal Test: Principle, Procedure, Results, Limitation

Widal Test: Principle, Procedure, Results, Limitation

Widal test is one of the oldest and most widely used serological tests for the diagnosis of enteric fever.

Clostridioides difficile: Characteristics, Disease, Lab Diagnosis

Clostridioides difficile: Characteristics, Disease, Lab Diagnosis

Gram positive obligate anaerobe responsible for a unique colonic disease, pseudomembranous colitis.

Elek Test: Principle, Procedure, Results

Elek Test: Principle, Procedure, Results

Elek test is an in vitro immunoprecipitation test to determine whether or not a strain of Corynebacterium diphtheriae is toxigenic.

Streptococcus Pyogenes (GAS): Characteristics, Diseases, Tests

Streptococcus Pyogenes (GAS): Characteristics, Diseases, Tests

This group A beta-hemolytic streptococci is bacitracin sensitive and causes strep sore throat, rheumatic fever, and PSGN.

Modified Oxidase Test (Microdase): Procedure, Uses

Modified Oxidase Test (Microdase): Procedure, Uses

Microdase test is a rapid method to differentiate Staphylococcus aureus (negative) from Micrococcus luteus (positive).

Tuberculin Skin Test (Mantoux test): Principle, Procedure, Results

Tuberculin Skin Test (Mantoux test): Principle, Procedure, Results

Tuberculin skin test is the standard method of determining whether a person is infected with M.tuberculosis or has exposure to tubercle bacilli.

Butyrate Disk Test: Principle, Procedure, Results, Uses

Butyrate Disk Test: Principle, Procedure, Results, Uses

Rapid test for identifying Moraxella (Branhamella) catarrhalis which produce butyrate esterase enzyme.

Endospore Staining: Principle, Procedure, Results

Endospore Staining: Principle, Procedure, Results

In Schaeffer-Fulton`s method, primary stain-malachite green is forced into the spore by steaming the smear, safranin is counterstain.

API 20E Test System: Results and Interpretations

API 20E Test System: Results and Interpretations

API-20E test strip (from bioMerieux, Inc.) is used to identify the enteric gram negative rods.

Complement Fixation Test: Principle, Procedure, Results

Complement Fixation Test: Principle, Procedure, Results

In the presence of specific Abs to an infectious agent, any complement in the system is bound, leaving no residual complement for rxn with Abs to the RBCs.

Auramine-Rhodamine Fluorochrome Staining

Auramine-Rhodamine Fluorochrome Staining

Auramine-rhodamine fluorochrome staining also known as "Truant method of staining", is used to visualize Acid-fast bacilli (AFB).

Lancefield Classification of Streptococci

Lancefield Classification of Streptococci

Beta hemolytic streptococci are arranged into twenty (20) groups A-U without I and J (i.e. A through H and K through V).

Tween 80 Hydrolysis Test: Principle, Procedure, Results

Tween 80 Hydrolysis Test: Principle, Procedure, Results

Mycobacterium species possess an enzyme-lipase, that splits Tween-80 and releases oleic acid and polyoxyethylated sorbitol.

Culture media for Salmonella typhi and paratyphi

Culture media for Salmonella typhi and paratyphi

Common culture media are bismuth sulfite agar, MacConkey agar, deoxycholate citrate agar, xylose lysine deoxycholate agar, salmonella-shigella agar, etc.

String test for Lab diagnosis of Vibrio cholerae

String test for Lab diagnosis of Vibrio cholerae

V. cholerae emulsified in sodium deoxycholate, it lyses the cell wall and releases DNA giving a mucoid “string” when a loop is drawn away.

DNase Test: Principle, Procedure, Results

DNase Test: Principle, Procedure, Results

DNase test determines the ability of an organism to hydrolyze DNA.

Hanging Drop Method for Bacterial Motility

Hanging Drop Method for Bacterial Motility

The drop hangs from the coverslip, and the petroleum jelly forms a seal that prevents evaporation. This preparation gives good views of microbial motility.

Specimen Collection for UTI Diagnosis

Specimen Collection for UTI Diagnosis

Clean-catch midstream urine, suprapubic aspiration and straight catheter techniques are common method of urine sample collection.

Gelatin Hydrolysis Test: Principle, Procedure, Results

Gelatin Hydrolysis Test: Principle, Procedure, Results

Gelatin hydrolysis test determines the ability of an organism to produce gelatinases that liquefy gelatin.

Bile-Esculin test for Enterococcus species

Bile-Esculin test for Enterococcus species

Bile-esculin test is widely used to differentiate Enterococci and Non-enterococcus group D streptococci from non-group D viridans group streptococci.

Ziehl-Neelsen Technique-AFB Staining

Ziehl-Neelsen Technique-AFB Staining

Ziehl-Neelsen acid fast stain is designed to stain bacterial cells containing long chain fatty (mycolid acids) such as Mycobacterium.

PYR Test: Principle, Procedure, Results

PYR Test: Principle, Procedure, Results

Some gram-positive cocci such as Enterococcus produces enzyme L-pyrroglutamylaminopeptidase that hydrolyze PYR substrate.

Trans-Isolate (T-I) Medium

Trans-Isolate (T-I) Medium

Tran-isolate medium is used to inoculate and transport CSF sample, If the CSF cannot be transported to a microbiology laboratory immediately.

Rose Bengal Plate Test (RBT) for Brucella

Rose Bengal Plate Test (RBT) for Brucella

Rose Bengal test is a simple, rapid slide-type agglutination assay performed with a stained B. abortus to diagnose brucellosis.

Brucellosis: Etiology, Pathogenesis, Lab Diagnosis

Brucellosis: Etiology, Pathogenesis, Lab Diagnosis

Brucellosis, also known as “Undulant fever”, “Mediterranean fever” or “Malta fever” or “Bang’s disease”  is a zoonosis caused by Brucella melitensis.

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Typhus Fever: Characteristics and Lab Diagnosis

Vibrio Cholerae: Characteristics, Pathogenesis, and Lab Diagnosis

Vibrio Cholerae: Characteristics, Pathogenesis, and Lab Diagnosis

Cholera is an acute infection of the gut caused by ingestion of food or water contaminated with the bacterium Vibrio cholerae O1 or O139.

Chlamydia Infections: Causes, Symptoms, Diagnosis, Treatment

Chlamydia Infections: Causes, Symptoms, Diagnosis, Treatment

Colony Morphology of Bacteria

Colony Morphology of Bacteria

A colony is defined as a visible mass of microorganisms. Colony characteristics of microorganisms help in their identification.

Sputum Sample: Collection, Transport, Staining, and Culture 

Sputum Sample: Collection, Transport, Staining, and Culture 

The first morning expectorated sputum sample is always best for the diagnosis of lower respiratory tract infections.

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Microbiology Specimen Collection Guidelines

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Blood Culture: Key points for Lab diagnosis

Modified Kirby-Bauer Disc Diffusion Method

Modified Kirby-Bauer Disc Diffusion Method

Modified Kirby-Bauer disc diffusion test method is a reference method to test the susceptibility of a bacterial isolate in a clinical laboratory.

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Antibiotics used in routine Antimicrobial susceptibility testing (AST)

What does Susceptible, Intermediate, and Resistant Mean?

What does Susceptible, Intermediate, and Resistant Mean?

Susceptible organism is likely to respond to treatment with this drug, at the recommended dosage but resistant wont respond.

How to store commercial antimicrobial discs

How to store commercial antimicrobial discs

Satellitism Test for Haemophilus influenzae

Satellitism Test for Haemophilus influenzae

Haemophilus spp may grow on sheep blood agar very close to the colonies of Staphylococcus aureus.

RPR Test: Principle, Procedure, Interpretations

RPR Test: Principle, Procedure, Interpretations

RPR test is a macroscopic, nontreponemal flocculation card test used to screen for syphilis. RPR test measures IgM and IgG antibodies to lipoidal material.

Anti-streptolysin O Titer: ASO Test

Anti-streptolysin O Titer: ASO Test

Rapid latex agglutination test for the qualitative and semi-quantitative determination of anti-streptolysin-O antibodies (ASO) in serum.

CSF Sample: Collection, Processing, Staining, and Culture

CSF Sample: Collection, Processing, Staining, and Culture

Common etiologic agents of acute meningitis are enteroviruses and bacteria (S. pneumoniae, Neisseria meningitidis, Haemophilus influenzae).

Haemophilus influenzae: Characteristics, Pathogenesis and Lab Diagnosis

Haemophilus influenzae: Characteristics, Pathogenesis and Lab Diagnosis

Gram-negative, oxidase-positive, coccobacilli, which requires X and V factors for the growth.

How to identify Streptococcus pneumoniae?

How to identify Streptococcus pneumoniae?

Gram positive diplococci (lanceolate shaped), alpha hemolysis, optochin sensitive and bile soluble isolates are S.pneumoniae.

Biochemical Tests to identify Mycobacteria, NTM

Biochemical Tests to identify Mycobacteria, NTM

Tests are; Tween-80 hydrolysis test, niacin test, nitrate reduction test, tellurite reduction test, urea hydrolysis test and citrate utilization test.

Neisseria gonorrhoeae: Disease, Pathogenesis, Lab Diagnosis

Neisseria gonorrhoeae: Disease, Pathogenesis, Lab Diagnosis

Neisseria gonorrhoeae is a gram-negative kidney-shaped diplococci. It causes gonorrhea, a sexually transmissted infection.

Laboratory diagnosis of Syphilis

Laboratory diagnosis of Syphilis

Syphilis is diagnosed by using dark field microscopy, non-treponemal tests (VDRL, RPR) or treponemal tests (TPHA, FTA-ABS).

FTA-ABS Test: Principle, Procedure, Results

FTA-ABS Test: Principle, Procedure, Results

FTA-ABS is an indirect fluorescent antibody technique used as a confirmatory test for syphilis. FTA-ABS is the first serological test to become positive following infection.

Chlamydia trachomatis: Properties, Disease, Lab Diagnosis

Chlamydia trachomatis: Properties, Disease, Lab Diagnosis

Chlamydia trachomatis, is an obligate intracellular bacteria which causes pelvic inflammatory disease and other infections (e.g. eye).

Shigella: Disease, Properties, Pathogenesis, Lab Diagnosis

Shigella: Disease, Properties, Pathogenesis, Lab Diagnosis

The causative agent of Bacillary dysentery, Shigella is a member of family Enterobacteriaceae. It is non-motile, non-lactose fermenter.

Campylobacter Jejuni: Disease, Properties, Lab Diagnosis

Campylobacter Jejuni: Disease, Properties, Lab Diagnosis

The most common cause of gastroenteritis worldwide, Campylobacter is a curved, gram-negative rod. It is an oxidase-positive microaerophile.

Optochin Sensitivity Test: Principle, Procedure, Results

Optochin Sensitivity Test: Principle, Procedure, Results

Optochin (ethylhydrocupreine hydrochloride) lyses pneumococci (positive) but alpha-streptococci are resistant (negative).

Bacitracin Test: Principle, Procedure, Results

Bacitracin Test: Principle, Procedure, Results

Bacitracin test differentiates S. pyogenes (inhibited) from other beta-hemolytic streptococci.

Streptococcus pneumoniae (pneumococcus): Disease, Properties

Streptococcus pneumoniae (pneumococcus): Disease, Properties

Pneumococcus is lanceolate shaped gram-positive cocci and causes pneumonia and meningitis. Its is optochin sensitive and bile soluble.

Culture Media: PH Indicators, Color of Colonies

Culture Media: PH Indicators, Color of Colonies

Characteristics pigmentation observed in the culture media is because of the changes in the pH of the medium.

Staphylococcus aureus: Disease, Properties, Lab Diagnosis

Staphylococcus aureus: Disease, Properties, Lab Diagnosis

Frequent colonizer of skin, S. aureus is a major cause of surgical wound infection. This gram-positive cocci is catalase and coagulase +ve.

Salmonella: Properties, Disease, Lab Diagnosis

Salmonella: Properties, Disease, Lab Diagnosis

Escherichia coli (E. coli): Properties and Identification

Escherichia coli (E. coli): Properties and Identification

Normal flora of GI tract, E.coli is a major cause of urinary tract infections. It is lactose fermenting, gram negative bacilli.

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Isolation of ESBL producing E. coli from Urine

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Collection and Transport of Stool Specimens

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Pseudomonas aeruginosa: Infections, Pathogenesis and Lab Diagnosis

Gram negative rods, catalase positive and oxidase positive. It produces pigments (pyocyanin and pyoverdin).

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Non-culturable Pathogenic Bacteria: Challenge to Lab Diagnosis

We can not culture T. pallidum, M. leprae, Chlamydia trachomatis, Coxiella burnetti and Rickettsia in in-vitro culture media.

Leprosy: Etiology, Pathogenesis, Lab Diagnosis

Leprosy: Etiology, Pathogenesis, Lab Diagnosis

Also known as Hansen’s disease, leprosy is a chronic infectious disease caused by an acid-fast bacillus, Mycobacterium leprae.

Coagulase Test: Principle, Procedure, Results

Coagulase Test: Principle, Procedure, Results

Coagulase test detects clumping factors and bound coagulase and differentiates Staphylococcus aureus (positive) from CoNS (negative).

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Isolation and Enumeration of Bacteria