Microbiology: Gram-Negative Rods Overview

Questions and Answers

What is the purpose of publishing an annual antibiogram according to CLSI recommendations?

To provide aggregate susceptibility testing results for organism-drug combinations (correct)

To establish a database for tracking infection rates

To demonstrate the effectiveness of all antimicrobial agents

To compare antibiotic costs among different hospitals

Which of the following microorganisms can be recovered through appropriate culture methods mentioned in the content?

Clostridium difficile

Mycobacterium tuberculosis (correct)

Escherichia coli

Staphylococcus epidermidis

Which treatment enhances the observation of yeasts and molds in specimens?

Treatment with KOH and calcofluor white (correct)

Culture on enriched media

Incubation at low temperatures

Application of Gram stain

In which situation is empiric therapy particularly significant?

When results of susceptibility tests are not yet available Signup and view all the answers

Which condition is commonly associated with yeast or fungus infections?

Eroded skin and mucous membranes Signup and view all the answers

What is the primary purpose of designing PCR primers in the context of microbial identification?

To hybridize to conserved genomic regions Signup and view all the answers

Which gene is most commonly used for bacterial identification due to its conserved regions?

16S rRNA Signup and view all the answers

What technique does MALDI-TOF MS employ to facilitate mass spectrometry analysis?

Ultraviolet (UV) irradiation Signup and view all the answers

In the context of mass spectrometry, what primarily characterizes the charged particles created by ionization?

Their charge-to-weight ratio Signup and view all the answers

What role does PCR-sequencing have in virology, according to the content?

Detection of specific resistance mutations Signup and view all the answers

Which of the following describes the application of MassTag PCR?

It incorporates a tag of known mass into the PCR product Signup and view all the answers

Which of the following is not identified as a conserved gene target for bacterial identification?

cytochrome c Signup and view all the answers

What is a key advantage of using the 16S rRNA gene for distinguishing bacterial species?

It contains conserved regions interspersed with variable sequences Signup and view all the answers

Which bacteria is characterized as coagulase-positive and usually β-hemolytic?

S. aureus Signup and view all the answers

In a clinical context, which testing method is more sensitive than culture for identifying specific infections?

Nucleic acid test Signup and view all the answers

What kind of flora is typically associated with mixed anaerobic and aerobic bacteria?

Mixed flora Signup and view all the answers

What type of white blood cells are indicated in infections caused by trachomatis in the absence of associated organisms?

Neutrophils (PMNs) Signup and view all the answers

Which of the following is NOT typically associated with Gram-negative diplococci?

S. aureus Signup and view all the answers

Which type of organisms are often strongly suspected as the cause of pneumonia due to their unusual presence in sputum?

Gram-negative rods Signup and view all the answers

In what type of infections does the source of infection play a significant role in creating a susceptibility test battery?

Community-acquired or nosocomial infections Signup and view all the answers

What does the Clinical and Laboratory Standards Institute (CLSI) provide recommendations for?

Drug testing based on organism type and specimen Signup and view all the answers

What is the proper way to report common organisms found in abdominal abscesses?

Report normal gastrointestinal microbiota Signup and view all the answers

Which statement about zone sizes of growth inhibition is accurate?

Zone sizes vary with the pharmacologic characteristics of different drugs. Signup and view all the answers

What should be done with yeasts present in small numbers within a specimen?

Identify and report them Signup and view all the answers

Why should other fungi not normally present in a specimen be reported?

They can cause significant clinical diseases. Signup and view all the answers

What is necessary to compare the zone sizes of different drugs acting on the same organism?

A common measurement standard Signup and view all the answers

Which specimen is typically used for diagnosing Enteric fever caused by Salmonella serovar Typhi?

Blood Signup and view all the answers

What type of flora is predominantly found in anaerobic and aerobic bowel abscesses?

Mixed flora Signup and view all the answers

On which agar is Vibrio cholerae specifically cultured when there is a clinical suspicion?

TCBS agar Signup and view all the answers

What is the expected colony morphology of Campylobacter jejuni on a smear?

Gull wing-shaped Gram-negative rods Signup and view all the answers

Which of the following statements is true regarding Shigella species?

They produce an alkaline slant, acid butt without gas or H2S. Signup and view all the answers

Which type of specimen is recommended for culturing Campylobacter species?

Stool Signup and view all the answers

In diagnosing cholecystitis, which of the following is typically found in the culture?

Gram-negative enteric aerobes and B. fragilis Signup and view all the answers

What is a distinguishing characteristic of non-lactose fermenting colonies on MAC/EMB agar?

They appear colorless or transparent. Signup and view all the answers

What is the gram stain appearance of Bacteroides fragilis?

Pink bacilli Signup and view all the answers

What is a common laboratory method used to identify Salmonella species in stool samples?

Hektoen enteric agar culture Signup and view all the answers

Study Notes

Bacterial Characteristics and Culture

Gram-negative rods: Primarily associated with enteric infections and gastrointestinal flora, indicative of potential pathogens.
Bacteroides fragilis: Notable for being a Gram-negative anaerobe typically found in mixed flora; significant in infections involving anaerobic bacteria.
Entamoeba histolytica: Considered in cases of mixed aerobic and anaerobic infections.

Clinical Conditions and Specimen Collection

Cholecystitis: Key specimens include bile; culture on BA, CA, and MAC/EMB needed for anaerobic and aerobic Gram-negative isolation, including B. fragilis.
Abdominal or perirectal abscess: Fluid sample reveals mixed flora; usually involves both aerobic and anaerobic bowel flora with more than five species often present.
Enteric fever (Typhoid): Blood, stool, or urine specimens cultured on BA, CA, MAC/EMB, and Hektoen to identify Salmonella serovar Typhi. Culturing multiple specimens is critical due to lactose-negative, H2S-positive characteristics.

Enteritis and Diarrheal Illnesses

Enteritis and bacterial diarrheas: Stool specimens assessed on MAC/EMB, Hektoen, and enteric agar.
Salmonella species: Identifiable through non-lactose fermenting colonies on TSI slants, producing gas and acid; prevalent in gastroenteritis cases.
Shigella species: Similar identification through TSI, characterized by an alkaline slant, acid butt without gas or H2S.
Campylobacter jejuni: Requires incubation at 42°C; identification of “gull wing–shaped” Gram-negative rods and oxidase-positive colonies.

Vibrio cholerae Detection

Thiosulfate-citrate-bile salts-sucrose (TCBS) agar: Important for identifying Vibrio cholerae in clinically suspected cases, which appear as yellow colonies.

Arthritis Testing

Synovial fluid culture: Typically tested on BA or CA for Staphylococcus aureus, identifiable as Gram-positive cocci in clusters.

Molecular Methods in Microbial Identification

16S rRNA gene sequencing: Commonly used for bacterial identification due to its conserved regions interspersed with variable sequences.
Mass spectrometry: Utilized for protein or DNA analysis; advanced techniques, including MALDI-TOF MS, revolutionize microbial identification processes.

Additional Bacterial Identification Techniques

PCR-sequencing: Enables strain typing and detection of resistance mutations in viruses.
Multi-target PCR: Incorporates mass tags for easier identification during mass spectrometry analysis.

Antimicrobial Susceptibility Testing

Susceptibility patterns: Identified based on the type of infection and microbiota present; recommended practices by the Clinical and Laboratory Standards Institute (CLSI).
Antibiotic zone sizes: Vary by drug characteristics; crucial for determining effective therapy options.

Fungal and Yeast Infections

Common fungal species: Includes Candida and Aspergillus; identified microscopically in suspicious areas and culturing practices adjusted for accurate results.
Enhanced observation techniques: Using KOH and calcofluor white aids in recognizing yeasts and molds effectively.

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Description

This quiz covers the characteristics and classifications of Gram-negative rods, focusing on common enteric bacteria like Escherichia coli and Bacteroides fragilis. Additionally, it explores mixed flora, anaerobes, and possible infections caused by these organisms, including Entamoeba histolytica. Test your microbiology knowledge with this insightful quiz.