Microbiology for Nurses: Classification of Prokaryotes and Key Microbial Groups

Module 3: Classification of Prokaryotes

Introduction

This module provides an overview of the classification of prokaryotes, focusing on medically important bacteria and archaea. It is designed for nursing students to understand the differences between major microbial groups, their structure, and their relevance to human health.

Chapter 7: Gram-Positive and Gram-Negative Bacteria, Archaea

Key Questions Addressed

• Dichotomous Key: How to use a dichotomous key to differentiate between Gram-positive and Gram-negative bacteria.

• Yeast vs. Molds: What are the key differences in structure and function between yeast and molds?

• Helminths and Parasites: How to differentiate between helminths and other parasites based on their structure and human infections.

Classification of Selected Prokaryotes

Overview

Bacteria are classified into various groups based on genetic, structural, and metabolic characteristics. The following table summarizes the main classes and their clinical significance.

Group	Selected Classes	Notes
Pseudomonadota (Proteobacteria)	Alphaproteobacteria Betaproteobacteria Gammaproteobacteria	Alphaproteobacteria: Includes Ehrlichia and Rickettsia (cause tick-borne diseases). Betaproteobacteria: Includes Bordetella (whooping cough), Zoogloea. Gammaproteobacteria: Includes Vibrio (cholera), Salmonella (typhoid), Escherichia (E. coli), Pseudomonas (opportunistic infections), Klebsiella (pneumonia).
Campylobacterota	Campylobacterales	Includes Campylobacter (gastroenteritis), Helicobacter (ulcers).
Cyanobacteria	Photosynthetic bacteria	Grow in aquatic environments; some fix nitrogen.
Chlamydiota	Chlamydiaceae	Includes Chlamydia trachomatis (trachoma, STDs), Chlamydophila pneumoniae (respiratory infections).
Bacteroidota	Bacteroides	Found in the mouth and large intestine; important in gut flora.
Spirochaetota	Spirochaetes	Includes Treponema pallidum (syphilis), Borrelia (Lyme disease).

Gram-Positive vs. Gram-Negative Bacteria

• Gram-Positive Bacteria: Have a thick peptidoglycan cell wall and retain the crystal violet stain (appear purple under microscope).

• Gram-Negative Bacteria: Have a thin peptidoglycan layer and an outer membrane; do not retain the crystal violet stain (appear pink/red).

• Dichotomous Key: A flowchart or decision tree used to identify bacteria based on observable traits (e.g., cell wall structure, shape, metabolic properties).

Examples of Medically Important Bacteria

• Neisseria: Causes gonorrhea and meningitis (Gram-negative).

• Klebsiella: Causes pneumonia and urinary tract infections (Gram-negative).

• Escherichia coli: Common gut bacterium; some strains cause food poisoning (Gram-negative).

• Staphylococcus aureus: Causes wound infections and food poisoning (Gram-positive).

• Streptococcus pyogenes: Causes strep throat and skin infections (Gram-positive).

Archaea

• Archaea: Prokaryotes distinct from bacteria; often found in extreme environments (e.g., high salt, temperature).

• Not typically associated with human disease but important in environmental microbiology.

Additional info:

• Classification is based on genetic sequencing, cell wall composition, metabolic pathways, and ecological roles.

• Understanding bacterial classification helps in diagnosis, treatment, and infection control in clinical settings.