Bacterial Classification and Characteristics

Bacterial Classification and Characteristics

• Gram-negative cocci: Includes Neisseria and Moraxella. Characterized by oxidase-positive and gram-negative diplococci. These bacteria are capnophilic, meaning they prefer environments with carbon dioxide. Common pathogenic species: N. gonorrhoeae and N. meningitidis.

• Neisseria: N. gonorrhoeae is a major sexually transmitted disease (STD) cause. N. meningitidis causes serious, life-threatening meningitis, sometimes with rash.

• Moraxella catarrhalis: Causes localized respiratory tract infections, including sinusitis and pneumonia, commonly affecting elderly patients and those with COPD (chronic obstructive pulmonary disease).

• Gram-negative bacilli: Include species within the Enterobacteriaceae family. These are non-spore forming, facultative anaerobes, and primarily motile but Klebsiella and Shigella are notable exceptions. Most are glucose fermenters, and catalase positive (except Shigella dysenteriae).

• Enterobacteriaceae: Many are motile at 37°C but species like Yersinia pestis are motile at 25°C but not 37°C. Species vary in whether they are rapid or slow lactose and/or sucrose fermenters. Many produce gas and H2S. Significant differences exist in their reactions to various biochemical tests (e.g., IMVIC, LIA, TSI, etc.). Critically, Shigella species are xylose non-fermenters.

• HACEK group: Include Haemophilus spp., Aggregatibacter actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella spp.. Known for fastidious nutritional requirements and need for increased CO2.

Other Gram-Negative Bacteria

• Vibrio, Aeromonas, Plesiomonas, Chromobacterium: These bacteria possess certain characteristics: curved comma shape, polar or peritrichous flagella, facultatively anaerobic, rapid darting/shooting star motility, and are oxidase and catalase positive. Except V. cholerae, species may be halophilic. V. cholerae causes cholera characterized by rice-water stool.

• Campylobacter: Microaerophilic, curved/sea gull-winged, faintly staining, and associated with gastritis and diarrhea. Optimum temp for growth is 42-43°C and these are motile with a single polar flagellum.

• Helicobacter: S-shaped, seagull wings, microaerophilic and associated with peptic ulcers. Rapid urease producer.

Non-fermenting Gram-Negative Bacilli

• Pseudomonas aeruginosa: An important opportunistic pathogen causing various community- and health-care-associated infections. Known for its characteristic odor, non-fermenter of glucose, and β-hemolysis. Growth at 42°C differentiates it from other Pseudomonas sp.

• Burkholderia:

• Burkholderia pseudomallei causes melioidosis. Characterized by bipolar staining and earthy oder.

• Burkholderia cepacia is another important species of the genus, associated with cystic fibrosis. Nonfastidious.

• Stenotrophomonas maltophilia: Often found in hospital settings, resistant to many drugs, and has a distinctive lavender-green colony. Oxidizes glucose and maltose strongly.

• Acinetobacter: A variety of species exist within the genus, notable for their differences in oxidase and carbohydrate utilization. Some species grow at 42°C. Differentiable species include Acinetobacter baumannii, with a reputation for drug resistance and frequent infections. Nonmotile.

Gram-Positive Bacilli and Other Bacteria

• Listeria monocytogenes: Characterized by a ability to cross the placenta, causing perinatal infections. It is a gram-positive rod, and motile at 22°C.

• Erysipelothrix rhusiopathiae: Causes veterinary infections and can be an occupational hazard for those who work with meat, poultry or fish. Gram-positive rod, nonmotile, and catalase negative, while producing H2S.

• Lactobacillus and Gardnerella vaginalis: Not typically pathogenic, although Gardnerella vaginalis is associated with vaginal discharge and "clue cells."

• Mycobacterium: Includes M. tuberculosis (tuberculosis) causing lung granulomas and Nontuberculous mycobacteria (NTM), that are found in the environment or can colonize humans.

• Actinomyces, Nocardia, Tropheryma whipplei, etc.: are distinct bacteria types.

• Chlamydia, Mycoplasma, Rickettsiae: These are obligate intracellular bacteria, with diverse pathogenic species and unique characteristics. C. psittaci infects birds and humans.

• Borrelia, Leptospira, Treponema: These are spirochetes, with significant pathogenic roles in relapsing fevers, Weil disease (Leptospirosis), and syphilis. Treponemal infections typically involve skin and mucous membranes. Transmission is often linked to sexual or non-sexual contact.

• Streptobacillus: Frequently associated with various fevers and also normal oral biota

• Haemophilus: Fastidious, gram-negative coccobacilli, needing X and V factors and capnophilic conditions (5% to 10% CO2 at 35°C to 37°C for up to 3 days).

• Brucella and Francisella tularensis: Important zoonotic pathogens, typically acquired through exposure to infected animals or their products.

• Legionella: Associated with water sources, especially in hospital settings, and L. pneumophila is a frequent cause of pneumonia in humans.

• Clostridia (anaerobic): This group includes C. perfringens, known for food poisoning; C. botulinum, associated with various forms of botulism through food or wound contamination; and C. tetani responsible for tetanus. Key characteristic is that they are gram-positive and anaerobes. These are noted for producing various toxins.

• Corynebacterium diphtheriae: Causes diphtheria, characterized by inflammation and pseudomembrane formation in the throat.