Acinetobacter: Characteristics and Diagnosis

• Acinetobacter is a genus of bacteria with a focus on characteristics, lab diagnosis, and role in nosocomial infections.

Learning Outcomes

• The characters and lab diagnosis of Acinetobacter must be identified.
• Acinetobacter can be recognized and identified as a cause of nosocomial infection.

Environmental Persistence

• Acinetobacter baumanii survives for months on clothing and bedclothes.
• Acinetobacter persists on beds, ventilators, and other hospital equipment.
• Acinetobacter is found on sink surfaces and doorknobs, making the spread of nosocomial infections difficult to control.

General Characteristics

• Acinetobacter are Gram-negative coccobacilli and are non-motile and non-spore forming.
• Acinetobacter is strictly aerobic, oxidase negative, and catalase positive.
• Acinetobacter are saprophytes and ubiquitous, can survive in nature and in the hospital, and survives on both moist and dry surfaces like human skin and mechanical ventilation equipment.

Diseases Caused by Acinetobacter

• Acinetobacter causes ventilator-associated pneumonia in ICU patients.
• Acinetobacter particularly effects catheterized patients, causing urinary tract infections.
• Acinetobacter causes bacteremia, especially in immunocompromised patients, leading to bloodstream infections.
• Acinetobacter causes surgical site and traumatic wound infections.

Lab Diagnosis: Culture

• On MacConkey's agar, Acinetobacter colonies appear pale yellow because they are non-lactose fermenting.
• Non-hemolytic colonies develop on blood agar within 24-48 hours.

Microscopic Morphology

• Gram-negative short coccobacilli appear in clusters or pairs.
• Cell arrangement shows pleomorphic variations in cell size and arrangement.
• Gram staining may show variable staining in some conditions.

Colony Morphology

• Colonies of Acinetobacter that grow on MacConkey agar are pale yellow and non-lactose fermenting.
• Acinetobacter colonies on blood agar are non-hemolytic with a smooth texture.

Colony Identification Methods

• Gram-negative coccobacilli with circular shape and entire margin can be identified through microscopy.
• A characteristic sardine odor helps in presumptive identification through olfactory assessment.
• A series of biochemical reactions confirm species identification through biochemical testing.

Classification of Acinetobacter

• The genus of Acinetobacter is subdivided into glucose oxidizing and glucose nonoxidizing species.
• Acinetobacter baumannii are glucose oxidizing species.
• Acinetobacter lowoffii and A. haemolyticus are glucose nonoxidizing species.

Glucose Metabolism Differences

• Acinetobacter baumannii ferments glucose and is a primary cause of nosocomial infections, classifying it as a glucose-oxidizing species.
• Acinetobacter Iwoffii and Acinetobacter haemolyticus do not ferment glucose, classifying it as a glucose non-oxidizing species.

Biochemical Identification

• Acinetobacter is oxidase negative, catalase positive, indole negative, non-motile, and negative for nitrate reduction.

Additional Biochemical Properties

• Acinetobacter species can be identified by decarboxylase activity, citrate utilization, methyl red test, and bile esculin test.
• Acinetobacter species are decarboxylase-positive.
• Citrate Utilization results can be positive or negative depending on the species.
• The Methyl Red Test is consistently negative across Acinetobacter species.
• A negative Bile Esculin Test helps differentiate from other bacteria.

A. baumannii Biochemical Profile

• A. baumannii is a glucose fermenter, resulting in positive Lysine Iron Agar (LIA) but negative Hydrogen Sulfide production reactions - carbohydrate metabolism.
• Enzyme activities of A. baumannii are negative for Indole, Oxidase, and Urease.
• A. baumannii has other properties of negative methyl red, citrase positive, and non-motile.

Treatment Challenges

• Multidrug-resistant Acinetobacter presents significant treatment challenges.
• Limited antibiotic options often require combination therapy and careful monitoring.

Emerging Treatment Options

• Combination therapy is used to overcome resistance mechanisms by using multiple antibiotics simultaneously.
• Polymyxins are a revival of older antibiotics like colistin, which are used as last-resort options.
• Development of new antimicrobial compounds targets resistant strains.
• Phage therapy constitutes the use of bacteriophages to target specific bacterial infections.

Key Takeaways

• Acinetobacter survives for months on hospital surfaces making the control of its transmission difficult.
• Acinetobacter are Gram-negative coccobacilli, oxidase-negative, with distinctive biochemical profile for identification.
• Acinetobacter has a major clinical impact as a cause of nosocomial infections, especially in ICU settings.
• Multidrug resistance generates significant treatment challenges requiring coordinated approaches.