Mycoplasma

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Introduction

  • Mycoplasma is the smallest free-living microorganism capable of self-replication.
  • It belongs to the class Mollicutes, meaning “soft skin,” because it lacks a rigid cell wall.
  • Mycoplasma is medically important because it causes respiratory, urogenital, and systemic infections in humans.
  • It is unique among bacteria because it can pass through filters that normally retain bacteria.
  • Due to the absence of a cell wall, Mycoplasma shows special staining properties and antibiotic resistance patterns.

Historical Background

  • First observed in 1898 during studies of pleuropneumonia in cattle.
  • Human pathogenic Mycoplasma species were identified later in the 20th century.
  • It became clinically significant after its role in atypical pneumonia was recognized.

General Characteristics

  • Extremely small size: 150–300 nm.
  • Pleomorphic: shape may be spherical, filamentous, or flask-shaped.
  • No true cell wall present.
  • Cell membrane contains sterols, which provide stability.
  • Gram stain is poor because of absent peptidoglycan layer.
  • Better visualized by Giemsa stain or special culture methods.
  • Facultative anaerobe in many species.
  • Slow-growing organism.
  • Genus: Mycoplasma
  • Key Species:
    • Mycoplasma pneumoniae (causes atypical pneumonia)
    • Mycoplasma genitalium (associated with urogenital infections)
    • Mycoplasma hominis (associated with pelvic inflammatory disease and other infections)
  • Family: Mycoplasmataceae
  • Gram Staining: Mycoplasma species do not have a cell wall, making them Gram-negative and resistant to standard Gram staining.
  • Shape and Arrangement:
    • Shape: Pleomorphic; can appear as spherical, filamentous, or branched forms.
    • Arrangement: Typically found as single cells but can form clusters.
  • Oxygen Requirements: Mycoplasma species are facultative anaerobes, some being strictly anaerobic.

Morphology

  • Mycoplasma is the smallest free-living bacterium known in microbiology.
  • Size ranges from 150–300 nanometers, so it is much smaller than most ordinary bacteria.
  • It is highly pleomorphic, meaning it can appear in many shapes because it does not have a rigid cell wall.
  • Common shapes seen are:
    • Spherical
    • Oval
    • Filamentous
    • Branched forms
    • Ring-shaped structures
  • The most important morphological feature is absence of cell wall.
  • Because there is no peptidoglycan layer, the organism is very flexible.
  • Instead of a cell wall, it has a three-layered cell membrane.

Cultural Characteristics

  • Mycoplasma is a fastidious organism, meaning it requires special enriched media for growth.
  • It cannot grow on ordinary nutrient agar because it lacks the ability to synthesize many essential nutrients.
  • Culture requires sterol-containing media, especially cholesterol, because sterols are essential for membrane formation.

Culture Media Used

  • PPLO medium (Pleuropneumonia-Like Organism medium) is the standard medium used.
  • Special enriched media contain:
    • Horse serum
    • Yeast extract
    • Cholesterol
    • Peptone
    • Glucose
    • DNA precursors

Why Serum is Necessary

  • Serum provides sterols required for membrane stability.
  • Without sterols, Mycoplasma cannot multiply.

Growth Conditions

  • Optimum temperature: 35–37°C
  • Optimum pH: 7.4–7.8
  • Requires humid atmosphere
  • Some species grow better in 5–10% CO₂

Growth Rate

  • Growth is very slow compared to ordinary bacteria.
  • Colonies usually appear after 2 to 7 days.
  • Some species may require longer incubation.

Colony Morphology

  • Colonies are extremely small and often need a dissecting microscope to observe.
  • Characteristic colony appearance is fried egg colony:
    • Dense central elevated zone
    • Flat translucent peripheral zone

Biochemical Reactions

  • Catalase Test: Mycoplasma species are catalase-positive.
  • Oxidase Test: Mycoplasma species are oxidase-negative.
  • Fermentation: Mycoplasma does not ferment carbohydrates; it requires preformed nutrients from the host or culture media.

Pathogenicity

  • Virulence Factors:
    • Adhesins: Surface proteins allow attachment to host cells.
    • Cytotoxic Effects: Mycoplasma can damage host cells and evade the immune response.
  • Clinical Infections:
    • Mycoplasma pneumoniae:
      • Causes atypical pneumonia, often called “walking pneumonia,” characterized by a persistent cough, fever, and malaise. It commonly affects younger populations.
    • Mycoplasma genitalium:
      • Associated with urethritis, cervicitis, and pelvic inflammatory disease. It can contribute to infertility.
    • Mycoplasma hominis:
      • It can cause urogenital infections and has been implicated in postpartum infections and other complications.

Laboratory Diagnosis

Specimen Collection

For Respiratory Infection

  • Throat swab
  • Nasopharyngeal swab
  • Sputum
  • Bronchoalveolar lavage

For Urogenital Infection

  • Urethral swab
  • Cervical swab
  • Vaginal swab
  • Urine sample

Direct Microscopy

  • Gram stain is not useful because Mycoplasma has no cell wall.
  • Organism cannot be clearly visualized by ordinary light microscopy.
  • Special stains may be used:
    • Giemsa stain
    • Dienes stain

Culture Method

  • Culture is done on PPLO medium (Pleuropneumonia-like organism medium).
  • Medium must contain:
    • Cholesterol
    • Horse serum
    • Yeast extract

Growth Features

  • Slow growth: colonies appear after 2–7 days
  • Colonies are very small
  • Typical fried egg colony seen under microscope

Colony Identification

  • Dense central part
  • Flat peripheral spreading zone

Serological Diagnosis

  • Most commonly used because culture is slow.

Important Serological Tests

  • Complement fixation test
  • ELISA
  • Indirect immunofluorescence

Cold Agglutination Test

  • A supportive diagnostic test in Mycoplasma infection.
  • Patient serum may contain cold agglutinins that agglutinate RBCs at low temperature.
  • Positive in many cases of Mycoplasma pneumonia.

Molecular Diagnosis

  • PCR is the most sensitive method.
  • Detects Mycoplasma DNA directly from specimen.
  • Useful in early infection.
  • Rapid and highly specific.

Advantages of PCR

  • Early detection
  • High sensitivity
  • High specificity
  • Useful before antibody formation

Antibody Detection

  • IgM antibodies appear early in infection
  • IgG antibodies indicate past exposure or late infection

Differential Diagnosis

  • Viral pneumonia
  • Legionnaires’ disease
  • Chlamydia pneumoniae infection

Important Laboratory Findings in Mycoplasma Pneumonia

  • Mild leukocytosis
  • Raised ESR
  • Chest X-ray may show patchy infiltrates

Diagnostic Gold Standard

  • PCR + serology combination gives best diagnosis

Antibiotic Resistance

  • Emergence of Resistance: Mycoplasma species show inherent resistance to beta-lactam antibiotics due to the absence of a cell wall.
  • Treatment Options:
    • Mycoplasma pneumoniae: Treatment typically includes macrolides (e.g., azithromycin) or tetracyclines (e.g., doxycycline).
    • Mycoplasma genitalium: Often treated with azithromycin or moxifloxacin, though resistance patterns are emerging.

Prevention

  • Prevention of Mycoplasma infection mainly focuses on interrupting transmission, because no effective vaccine is available at present.
  • Mycoplasma pneumoniae spreads mainly through respiratory droplets, especially in crowded places.

General Preventive Measures

  • Maintain proper respiratory hygiene.
  • Cover mouth and nose while coughing or sneezing.
  • Use tissue or elbow during coughing.
  • Dispose of used tissues properly.

Hand Hygiene

  • Frequent hand washing with soap and water.
  • Alcohol-based hand sanitizer can also be used.
  • Hand hygiene reduces indirect transmission.

Avoid Close Contact

  • Avoid close contact with infected individuals.
  • Maintain distance in outbreaks, especially in hostels, schools, and hospitals.

Use of Masks

  • Face masks reduce droplet spread.
  • Especially useful during respiratory outbreaks.

Isolation of Infected Patients

  • Patients with active respiratory symptoms should avoid crowded environments.
  • Isolation helps reduce community transmission.

Environmental Measures

  • Ensure good room ventilation.
  • Avoid overcrowding in classrooms, hostels, and wards.
  • Proper air circulation reduces organism spread.

Early Diagnosis and Treatment

  • Early detection allows rapid treatment.
  • Antibiotic therapy reduces duration of infectivity.

Prevention in Hospitals

  • Follow infection control precautions.
  • Separate symptomatic patients when possible.
  • Use proper specimen handling in laboratory.

Prevention of Urogenital Mycoplasma Infection

  • Safe sexual practices are important.
  • Screening of symptomatic patients may prevent spread.

Vaccine Status

  • Currently no licensed vaccine is available for Mycoplasma infection.
  • Prevention depends mainly on public health measures.

Outbreak Prevention

  • Important in:
    • Schools
    • Colleges
    • Military barracks
    • Hostels
    • Hospitals

Health Education

  • Educate patients about cough etiquette.
  • Encourage early medical consultation for persistent cough.
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