Chapter 13

Microbial Life

Characteristics of MicrobesProkaryotic cells

Smaller Lack special structures such as a nucleus and

organellesAll prokaryotic cells are microorganisms

Some microorganisms are eukaryoticViruses?

“Micro”organisms

Pathogens causing disease

Normal / indigenous floradecomposing dead skin cells supplying essential vitamins, guarding against pathogenic

organisms Saprobes

In soil decompose dead organisms,

sustaining chemical cycles

Microorganisms are diverse and widespread

Microbiology The study of organisms (microorganisms

or microbes) too small to be seen without magnification

This includes:1. Bacteria2. Viruses3. Fungi4. Protozoa 5. Algae6. Helminthes

Bacteria

Bacteria– Single-celled

organisms – Various shapes

• Cocci• Bacilli• Spiral shapes

– Cellular– Lack membrane-

enclosed cellular structures

– Widely distributed in nature

Generalized structure of a prokaryotic cell

Appendages

Flagellarotates 360o

1-2 or many distributed over entire cell

Fimbriaeadhesion

Pilimade of pilin proteinfound only in Gram negative cellsFunctions

joins bacterial cells for DNA transfer (conjugation)

Adhesionto form biofilms and

microcolonies

The Cell Envelope External covering outside the cytoplasm Composed of few basic layers:

glycocalyx cell wall

Gram positive Gram negative

cell membrane Maintains cell integrity

Chromosome

single, circular, double-stranded DNA molecule

contains all the genetic information required by a cell

DNA is tightly coiled around a proteindense area called the

nucleoid

Plasmids

small circular, double-stranded DNA

stable extrachromosomal DNA elements that carry nonessential genetic information

duplicated and passed on to offspringreplicate independently from

the chromosome

Endospores

resting, dormant cellsproduced by some G+ generaresistance linked to high

levels of calcium & certain acids

longevity verges on immortality 25 to 250 million years

pressurized steam at 120oC for 20-30 minutes will destroy

Viruses

Viruses

– Acellular– Composed of

nucleic acid and a few proteins

– Requires electron microscope for viewing

General Structure of VirusesCapsids

All viruses possessConstructed from identical

subunits called capsomers made of protein

Structural types: helical

Continuous helix of capsomers forming a cylindrical nucleocapsid

icosahedral 20-sided with 12 corners

vary in the number of capsomers

General Structure of Viruses

Viral envelopemostly animal virusesacquired when virus leaves host cellProtects the nucleic acid when the

virion is outside the host cellspikes

exposed proteins on the outside of the envelope

essential for attachment of the virus to the host cell

Nakedcomposed only of a nucleocapsid

Envelopedsurrounded by an envelope

Nucleic acidsViral genome

either DNA or RNA but never bothNumber of genes varies for each type of virus

few to hundredsDNA viruses

usually double stranded (ds) may be single stranded (ss)circular or linear

RNA viruses usually single strandedmay be double strandedSegmented versus nonsegmented

6 Steps in Viral Replication

1. adsorption 2. penetration 3. replication 4. assembly5. maturation6. release

Fungi

Fungi– Yeasts and molds

• Single-celled, microscopic– Mushrooms

• Multicellular, macroscopic– Cell nucleus and other

cellular structures– Absorb nutrients from their

environment– Saprobes

– Widely distributed in water and soil

Thermal dimorphism grow as molds at 30°C and

as yeasts at 37°C

Fungal Organization - Mold

Fungal Cell StructureHyphae

MyceliumConidia / sporesSexual and asexual

reproductionCell wallsCell walls contain chitin Energy reserve is glycogen Nonmotile

Produce wind-blown sporesGrow toward food source

Fungal Organization

Yeasts Soft, uniform texture

and appearance Unicellular (bicellular)False hyphaeBeta-glycan cell wall

structure

Protozoa

Protozoa100,000 species

@ 25 are important pathogensSome spread by insect vectors

Vary in shape, lack a cell wall Most are unicellularColonies are rareObtain food by engulfing or ingesting

smaller organisms and plant materialMost are harmless, free-living in a moist

habitat

Protozoa

Exist as trophozoite motile feeding stage

cystDormant resting stage

when conditions are unfavorable for growth and feeding

All reproduce asexually, mitosis or multiple fission

Many also reproduce sexually

Protozoan Classification

Simple grouping is based on method of motility, reproduction, and life cycle

Mastigophora primarily flagellar motility

Sarcodina primarily ameba

Ciliophora Cilia

Apicomplexa motility is absent except male gametes

Algae

Algae– Unicellular to

multicellular – Macroalgae– Microalgae

– Have a nucleus and many membrane-enclosed cellular structures

– Photosynthesize their own food

– Widely distributed in fresh and salt water

– Important source of food for other organisms

Algae DiversityDiatoms

single-celled silica shell

Euglenophyta single-celled photosynthesis

Dinoflagellata flagella and can moveusually photosyntheticsometimes

phosphorescentplankton

Chrysophyta silica in their cell wall found in fresh water

Phaeophyta contains many

familiar seaweeds.Rhodophyta

red pigmentscontains many types

of seaweed