Methods of Microbiology

Methods of Microbiology
• Increase contrast of microorganisms
• Classified into types of stains
– Simple stain:
– Differential stain:
– Structural or special stains
• Organic salts with positive and negative
• One ion is colored –chromophore
• Basic dye: positive ion is colored
– MeBlue+
• Acidic dye: negative ion is chromphore
Basic Dye
• Works best in neutral or alkaline pH
• Cell wall has slight negative charge at pH
• Basic dye (positive) attracted to cell wall
( negative)
• Crystal violet, methylene blue, safranin
Acidic Dye
• Chromophore repelled by negative cell
• Background is stained, bacteria are
• Negative stain-look at size, shape
• Eosin, India ink
Simple Stains
• One dye, one step
• Direct stain using basic dye
• Negative stain using acidic dye
Differential Stains
• More than one dye
• Gram stain, acid fast
• Primary dye
• Decolorizing step
• Counter stain
Special/ Structural Stains
• Identify structures within or on cells
• Different parts of cell are stained different
• Culture media
• Inoculum
• Culture
• Pure culture
• Million cells to be visible
Living vs Nonliving
• Viruses, few bacteria
• Living host-eggs, tissue cells
• Mycobacterium leprae –armadillos
• Most microbes grow on nonliving media
Synthetic or Chemically Defined
• Exact chemical composition known
• Chemoheterotrophs
– Glucose-carbon source & energy source
– Supplies chemical requirements
Complex Media
Used for most chemoheterotrophs
Bacteria and fungi
Cannot write formula for each ingredient
C,N,energy, S requirements
Vitamins, other growth factors
Complex Media
• Nutrient broth –liquid form
• Nutrient agar –solid form
– Plate
– Deep
– Slant
Anaerobic Methods
• Reducing media
• Anaerobic jar
• Use both in lab
Candle Jar
• Reduce oxygen levels
• Provides more CO2
• Microaerophilics
Selective and Differential Media
• Selective
• Differential
• Passage of liquid through screen device
• Pores small enough to retain microbes
• Sterilize heat sensitive materials
• Negative
• Positive
• HEPA hoods & TB rooms
• Uses temperature above boiling water
• Steam under pressure
• Preferred method unless material is
• Higher the pressure, higher the
• Need direct contact with steam
• 15 psi at 121 C for 15 mins
• Prions- protein only
• Flash sterilization-at 134 C for 3min
• Packaging
• Use of indicators
Compound Microscope
• Assigned scope
• Know parts & functions
• Proper use & care of scope
• Use of light ( visible or UV) or electrons
• Lenses to magnify
• Total magnification of compound scope
• Parfocal
• Working distance
• Ability to distinguish 2 objects as separate
and distinct
• Dictated by physical properties of light
• Limit is 0.2um for our light scope
Light Scope
• Simple vs compound
• Source of illumination
• Visible light has average wavelength of
550nm or 0.55 um
– Enters condenser lenses
– Focused into a cone
Light Path
• Passes through opening in stage to slide
• Light enters objective lens
– Image magnified by ocular lens
• Improves image detail
• Difference in light intensity
• Bacteria are colorless
• Need to increase artificially by staining
• Contrast is property of specimen
• Distinguish detail within image
• Property of lens system, measured as
resolving power
• Closest that 2 points can be together and
still seen as separate
• RP = wavelength of light
2 X NA
Resolving Power
• Function of numerical aperture: NA
• Function of wavelength of light
• Refractive index of material between
objective lens & specimen
Oil Immersion Lens
• Light bends (refracts) as it passes from
glass into air
• Use oil between slide and 100x lens
• Increases resolution
Oil Immersion Lens
• Lens captures more light
• Shortest working distance
• Summary: increased resolution
Fluorescent Microscope
• Used to view antigen antibody reactions
• Specimen tagged with fluorescent dye
• Ocular lens fitted with filter that permits
longer wavelengths & blocks shorter ones
• UV light(230-350nm)
Electron Microscopy
• Uses electrons as source of illumination
• Wavelength of electrons is dependent
upon voltage of electron beam