Biochemical Identification of Gram-Negative Bacteria

Questions and Answers

What is the primary purpose of the citrate test?

To identify hydrolysis of DNA

To assess the ability to use sodium citrate as a sole carbon source (correct)

To determine nitrogen source utilization

To evaluate gelatin breakdown

What color change indicates a positive citrate utilization test in Simmons' citrate medium?

From red to brown

From green to blue (correct)

From yellow to pink

From clear to cloudy

Which bacterial species is known to produce extracellular DNase?

Bacillus subtilis

Streptococcus pyogenes

Staphylococcus aureus (correct)

Escherichia coli

What is the result of adding 1N HCl to a DNase test medium after incubation?

Dissolution of oligonucleotides Signup and view all the answers

What does gelatinase activity indicate when gelatin is liquefied?

Proteolytic enzyme production Signup and view all the answers

Which factor can influence the activity of gelatinase?

Size of inoculum and incubation temperature Signup and view all the answers

What does the presence of indole indicate in a bacterial test?

Degradation of tryptophan Signup and view all the answers

Which of the following mediums is used to test for gelatin liquefaction?

Gelatin medium Signup and view all the answers

What is an intermediate degradation product formed when tryptophan is deaminated by tryptophanase?

Indole Signup and view all the answers

Which method is more sensitive for detecting indole?

Ehrlich indole test Signup and view all the answers

What color change indicates a positive malonate utilization test?

From green to blue Signup and view all the answers

How is motility primarily determined in bacteria?

By observing growth in a semisolid medium Signup and view all the answers

What is the function of Bromthymol blue in malonate broth?

To act as a pH indicator Signup and view all the answers

What reagent is added to the tube in the Ehrlich indole test to develop a red color indicating the presence of indole?

PDAB (para-dimethylaminobenzaldehyde) Signup and view all the answers

Which of the following options is true regarding the rapid indole test?

Isolated colonies are smeared onto moistened filter paper. Signup and view all the answers

What nitrogen source is commonly used by bacteria that can utilize sodium malonate?

Ammonium sulfate Signup and view all the answers

What does the binary code represent in the identification process of organisms?

The identifying phenotype of specific organisms Signup and view all the answers

How has the number of genera within the Enterobacteriaceae family changed from the early 1960s to 2003?

Increased from 10 to 31 Signup and view all the answers

Which component is NOT a part of the API 20E identification system?

Incubation at 25°C Signup and view all the answers

What is the purpose of the oxidase test in the API 20E system?

To determine the profile number Signup and view all the answers

What is the duration of incubation for the API 20E system?

18 to 24 hours Signup and view all the answers

What does the code profile number assist with?

Cross-referencing with a known organism database Signup and view all the answers

What type of bacteria is the API 20E system primarily designed to identify?

Gram-negative fermentative bacteria Signup and view all the answers

What supplemental tests can be used if the identification via code profile is uncertain?

Nitrate reduction and motility tests Signup and view all the answers

What color indicates a positive reaction in Kovac's oxidase test?

Lavender Signup and view all the answers

Which component is primarily responsible for color change in the urease test?

Phenol red Signup and view all the answers

In lysine iron agar, what does a dark purple butt indicate?

Lysine decarboxylation Signup and view all the answers

What type of reaction does urease catalyze?

Hydrolysis of urea Signup and view all the answers

What is the primary use of lysine iron agar?

Screening for enteric pathogens Signup and view all the answers

Which of the following is true about ρ-Aminodimethylaniline oxalate compared to tetramethyl-ρ-phenylenediamine dihydrochloride?

It is less sensitive Signup and view all the answers

What happens to the color of urease media after a microorganism hydrolyzes urea?

Changes to bright pink Signup and view all the answers

Which of the following is a key component of lysine iron agar?

Sodium thiosulfate Signup and view all the answers

What is the reported accuracy of identifying commonly isolated Enterobacteriaceae after 24 hours of incubation?

87.7% Signup and view all the answers

Which system is NOT mentioned as a multitest system for Enterobacteriaceae?

BioMérieux API 20 Signup and view all the answers

What was the primary method used by microbiologists for bacterial identification until the late 1970s?

Growth and isolation in broth culture Signup and view all the answers

What is the effect of rapid and accurate reporting of laboratory results on patient care?

Improved clinical outcomes Signup and view all the answers

Which of the following statements about the MicroLog systems is TRUE?

They are both manually and semiautomatically read. Signup and view all the answers

What is a significant challenge in clinical microbiology laboratories today?

Recognition of old pathogens in new clinical settings Signup and view all the answers

Why is rapid reporting increasingly important in clinical microbiology?

It addresses challenges of emerging and resistant organisms. Signup and view all the answers

What is the accuracy rate for less commonly isolated Enterobacteriaceae at 24 hours of incubation?

78.7% Signup and view all the answers

Study Notes

Miscellaneous Tests for Biochemical Identification

Citrate Utilization: Utilizes Simmons' citrate medium, which contains sodium citrate as a carbon source and ammonium salts as nitrogen. A positive result is indicated by a color change from green to blue due to increased pH from ammonia production. Christensen's medium, with phenol red, is an alternative that turns yellow to pink.
DNase Test: Detects the presence of DNase enzymes that cleave DNA. Medium contains 0.2% DNA; a clear zone forms after incubation with HCl if DNase is produced. Common DNase producers include Staphylococcus aureus and Serratia marcescens.
Gelatin Liquefaction: Measures gelatinase activity, where bacteria produce enzymes to degrade gelatin into amino acids. Liquefaction indicates positive gelatinase activity; incubation may take weeks for results.
Indole Production: Indicates the presence of tryptophanase, which converts tryptophan to indole. Detected using the Ehrlich or Kovac's reagent, resulting in red coloration if indole is present. The rapid indole test can give results in under 2 minutes.
Malonate Utilization: Assesses whether bacteria use sodium malonate as their sole carbon source. Changes indicator color from green to blue due to increased alkalinity from ammonium sulfate utilization.
Motility Test: Determines bacterial movement using semisolid media or microscopy. Kovac's oxidase test uses a reagent that turns lavender in the presence of oxidase enzymes, indicating positive results.
Urease Test: Evaluates the ability to hydrolyze urea and produce ammonia, leading to a pH change. A bright pink color indicates a positive urease reaction, typically tested in Christensen's urea agar.
Lysine Iron Agar (LIA) Slant: Used to detect lysine decarboxylation and H2S production; produces a purple butt for decarboxylation and black precipitate for H2S in alkaline conditions. Useful for differentiating Enterobacteriaceae.

Analytical Profile Index (API)

Developed in 1970 for identifying gram-negative fermentative bacteria (API 20E). The system consists of 20 cupules with substrates primarily for metabolic tests.
A bacterial suspension is used to hydrate the media, followed by incubation. Results generate a 7-digit code to identify species through a manufacturer database.
Known for 87.7% accuracy for common Enterobacteriaceae at 24 hours. Other multitest systems include Crystal E/NF, RapID, Microbact, and MicroScan. Biolog offers advanced systems with extensive species databases.

Rapid and Automated Identification Systems

Historically, diagnosis relied on culture methods, requiring 24-48 hours for isolation and identification. Rapid diagnostics are essential for improving patient outcomes by enabling early diagnosis and treatment.
The increasing threats from emerging pathogens and multidrug-resistant organisms necessitate advancements in identification methodologies for effective clinical microbiology practice.

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Description

This quiz focuses on the biochemical identification of Gram-negative bacteria, specifically examining the citrate utilization test. You will learn about Simmons' citrate medium and how bacteria can metabolize sodium citrate as a carbon source, leading to changes in pH. Test your knowledge on this important microbiological concept.