Biol 2051

• Exam 1 is Tuesday Sept 15th – 40 MC questions – Same room and time as lecture – You must have student ID and pencils – Do not bring a scantron

• Office hours from now until exam: – Tues Sept 8th 10:30-11:30 – Wed Sept 9th 8:00-9:00 and 1:30-2:30 – Thurs Sept 10th 8:00-9:00 and 10:30-11:30 – Mon Sept 14th 8:00-9:00 and 1:30-2:30 – Tues Sept 15th 8:00-9:00

Bacterial Cell Surface Structures

Pili & Fimbriae • Bacteria may have 1, both, or neither • Fimbriae: – Non-motile extensions that help bacteria attach to surfaces and to other bacteria (Neisseria, biofilms) – Shorter than flagella, may have 100’s per cell • Pili: – aka- conjugation pili – Hollow, non-motile tubes made of protein called pilin that connect some cells. – Longer than fimbriae, shorter than flagella; may have 1-10 per cell – Used to move DNA from 1 cell to another by conjugation

E. coli

capsule/slime layer/ glycocalyx • Sticky polysaccharide or polypepetide layer surrounding cell • Protects cell from: – phagocytosis – desiccation • Help cells attach to objects such as teeth

S. mutans

S. pneumoniae

Cell Motility • Flagella – Long, helical protein filaments – Attached at ends, or over whole cell - Flagella rotate to propel cell Proton passage drives rotation - Clockwise or counterclockwise

Bacterial flagella rotate; Eukaryotic flagella- whip-like motion

http://video.google.com/videoplay

Arrangement of Flagella • Monotrichous- single flagellum at 1 end • Lophotrichous- several flagella at 1 or both ends • Peritrichous- several flagella all around cell • Amphitrichous- 1 on each end peritrichous

lophotrichous

monotrichous

monotrichous

amphitrichous

lophotrichous

peritrichous

Structure of the flagella 3 parts: 1. Basal body 2. Hook 3. Filament

• Basal Body – Imbedded within cell envelope – Made of 2 or 4 protein rings connected by a central rod • C ring- in G+ & G• MS ring- in G+ & G• P ring- in G- only • L ring- in G- only outer membrane

L ring P ring

peptidoglycan

MS ring C ring

MS ring

cytoplasmic membrane

Gram-negative Bacterium

C ring

Gram-positive Bacterium

• C ring- In cytoplasm. Attached to inner surface of cytoplasmic membrane • MS ring- In cytoplasmic membrane. End of central rod is attached to MS ring. • P ring- In peptidoglycan layer • L ring- In LPS layer

outer membrane

L ring P ring

peptidoglycan

MS ring

MS ring

cytoplasmic membrane

C ring

Gram-negative Bacterium

C ring

Gram-positive Bacterium

• Hook – Curved structure made of protein; connects filament to basal body • Filament – Long, rigid, helical structures made of protein called flagellin

•Prokaryotes such as filamentous cyanobacteria, Myxococcus, Cytophaga & Flavobacterium move by gliding motility instead of flagella. •Gliding can occur from slime secretion that moves cell along solid surface.

cyanobacterium Oscillatoria

Flavobacterium

Motile bacteria can respond to chemical & physical gradients in environment by moving toward or away from the signal molecule.

•Directed movements toward or away from a chemical or physical signal are known as taxes. •Chemotaxis – directed movement of organisms in response to chemical signals. •Phototaxis – directed movement of organisms in response to light. •Aerotaxis – directed movement of organisms in response to oxygen. •Osmotaxis - directed movement of organisms in response to ionic strength.

Demonstration of chemotaxis

Initial insertion of capillary

Attractant

Neither attractant nor repellent

Repellent

0 hr

2 hr Phototrophic bacterium Rhodospirillum moving toward light

• Attractants cause counterclockwise rotation – Flagella bundle together – Push cell forward • “Run” • Repellents cause clockwise rotation – Flagella fly apart • “Tumble” = change of direction

• Runs + tumbles cause “random walk” – Receptors detect attractant concentrations • Sugars, amino acids – Attractant concentration increases and prolongs run • Net movement of bacteria toward attractants

Chapter 4 Bacterial Culture, Growth, and Development

Microbial Nutrition • All life requires: – Electron flow, to drive all life processes • Drives ions into, out of cells • Used to create ATP – Energy, to move electrons – Materials, to make cell parts • Nutrients- CHONPS

• Electron flow requires: – Source of electrons • Lithotrophs– Inorganic molecules are electron donors (iron) • Organotrophs– Organic molecules are electron donors (glucose) – Ultimate electron acceptor • Inorganic molecules (nitrate or oxygen) – Respiration • Organic molecules (pyruvate) – Fermentation

• Source of energy – Phototrophs • Light energy excites electrons • Excited molecules are electron donors – Chemotrophs • Chemicals are electron donors • Oxidation of chemical – Oxidation = donation of electrons

• Nutrients – Macronutrients • Major elements in cell macromolecules –C, H, O, N, P, S • Ions necessary for protein function –Mg2+, Ca2+, Fe2+, K+ – Micronutrients • trace elements (Co, Cu, Zn, etc) • growth factors (organic compounds) necessary for enzyme function

• Carbon- large amount needed by cells to form organic compounds (amino acids, fatty acids, sugars, & nitrogenase bases) to carry out cellular functions. • Autotrophs- prokaryotes that can make all cellular structures from CO2. • Heterotrophs- must obtain carbon from organic compounds. (most prokaryotes)

• Nitrogen- needed by cells for amino acids, nitrogen bases, & several other cell constituents. • Nitrogen-fixing prokaryotes- capable of using atmospheric nitrogen gas. • Most prokaryotes obtain nitrogen from compounds such as ammonia & nitrate.

Energy sources: Chemoorganotrophs -energy from oxidation (removing electrons) of organic compounds Chemolithotrophs – energy from oxidation of inorganic compounds. Only in prokaryotes. Advantage? Phototrophs - contain pigments that allow them to use light as an energy source. Advantage?

Carbon sources: Heterotrophs - carbon source is organic carbon compounds Autotrophs - carbon source is carbon dioxide These terms can be combined to more completely describe an organism. • Examplephotoautotroph obtains energy from light & carbon from carbon dioxide.

Nutrient Uptake • Passive diffusion – Some substances pass freely through membranes • O2, CO2 – Follows gradient of material • Facilitated diffusion – Transporters pass material into/out of cell – Follows gradient of material

Nutrient Uptake—Active Transport • ABC Transporters – Use ATP energy to pass material into cell – Transport material against gradient

• Symport and Antiport – Gradient of one molecule transports another – Transports material against its gradient Symport: Gradient of pumps in same direction

Antiport: Gradient of pumps in opposite direction

Phosphotransferase

System (PTS) Uses ATP energy to pass material into cell

glucose enters cell and is phosphorylated. As a result, gradient of pushes more glucose inside. (glucose-6-phosphate) cannot pass out of cell.

Nutrient Uptake—Active Transport Phosphotransferase

System (PTS) Uses ATP energy to pass material into cell Modifies material as it enters cell glucose enters cell and is phosphorylated. As a result, gradient of pushes more glucose inside. (glucose-6-phosphate) cannot pass out of cell.

Culturing Bacteria • Culture media-all materials necessary for growth – Varies for different bacterial species – Electron source – Energy source • If not phototrophic – Carbon source • If not autotrophic – Nitrogen source • If not N2-fixer

Obtaining Pure Cultures • Dilution streaking – Streak cells on plate – All cells in colony derive from single cell • Genetically identical

• Dilution in liquid culture – Reduces number of cells in each tube – Spread liquid on plate to see single colonies

Counting Bacteria Total Counts/Direct counts • Petroff-Hauser counting chamber – viewed under microscope & cells in grids are counted – Counts cells directly

• Can be done electronically using Coulter Counter • Can’t tell if cells are alive or dead – Can use special stains to distinguish living cells

Spectrophotometer/Turbidity measurements • Measures optical density • indirect but rapid • a suspension of cells looks turbid (cloudy); cells scatter light passing through suspension

• more cells, more turbid, more light is scattered • can’t tell if cells are alive or dead light bulb

Photodetector

• For turbidity measurements to be substituted for direct counting methods a standard curve must be made. • Once a standard curve is made for a specific organism growing in a specific culture medium, it can be used for future cultures of the same organism in the same medium to estimate cell numbers.

Viable counts • In many cases, you don’t want to count dead cells, so viable count methods let you count only live cells. – Counts only cells able to reproduce • Form colonies – Requires time to form colonies (overnight)

• Concentration of bacteria in a sample is unknown. • Before spread plates or pour plates are done, dilution of sample is necessary.

• Why study microbial growth? • to understand science of microbial growth • practical situations which call for control of microbial growth: – Food industry, health care industry, etc

• When 1 cell divides to form 2 cells, one generation has occurred. • Generation time- time for # of cells in a culture to double. • Many bacteria have generation time of 1-3 hours. Some as little as 10 minutes, some can be days. • Generation time is affected by nutritional & genetic factors. • Under ideal conditions, one generation in Escherichia coli takes 20 minutes.

• Also called doubling time because with each generation the cell population doubles • Generation time in lab is usually shorter than in nature. Why? • constant ideal conditions for lab cultures; natural populations rarely have ideal conditions

• http://video.google.com/videoplay?docid=-

•exponential growthcharacteristic type of growth pattern of microbial populations where the number of cells doubles over a regular time interval

•Exponential growth can be represented on a semilogarithmic graph •These graphs are useful for estimating generation times

Graphical determination of generation time • Number of cells/ml is plotted vs time on semi-log paper • Semi-log paper- linear scale on X-axis & logarithmic scale on Y-axis • Generation time is found by determining the time it takes the # of cells to double

•Each cycle on the Y-axis represents a power of 10 Example: •bottom 1 might represent 106 cells/ml, then next 1 would represent 107 •Or bottom 1 might represent 0.001 and next 1 would represent 0.01 •Depends on the data you have •Important to label the axes •Can plot # of cells or optical density/absorbance (turbidity)

Use the following data to plot growth curves and calculate generation time graphically X-axis- time Y-axis- # of cells

Cells/ml

1.5 x 106 1.5 x 106 1.5 x 106 2.0 x 106 4.5 x 106 1.3 x 107 4.5 x 107 2.2 x 108 1.0 x 109 2.8 x 109 4.5 x 109 5.5 x 109 6.2 x 109 7.0 x 109 8.0 x 109

Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

absorbance

.003 .003 .004 .008 .014 .033 .085 .180 .410 1.20 3.00 5.00 8.00 9.00 9.00

Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Generation time is ~ 30 minutes

Generation time is ~ 42 minutes

The Growth Cycle Populations of microorganisms show a characteristic growth pattern when inoculated into a fresh culture medium Log scale necessary to show wide range of concentrations

Phases of Growth

• • • •

Lag phase Exponential (logarithmic) phase Stationary phase Death phase

Growth curve = graph of # of cells vs time

Lag phase • cells are transferred to a new medium, delay before cells divide • No increase in cell # • Adjustment period (cells are making everything they need to grow in the new medium)

Exponential growth phase

• Also called logarithmic (or log) phase • Increase in cells is geometric – 1 cell will become 2, then 4, then 8, then 16, etc.

• Shortest generation time – time it takes for number of cells in a culture to double

Stationary phase • key nutrient will run out or toxic waste product will build up • Most cells survive but stop dividing

Death phase • Nutrients run out & waste products build upcells can no longer survive • Cells die & then may lyse (break apart)

Batch culture vs continuous culture/chemostat Batch culture• Constant volume of culture medium; closed system- nothing added or removed; commonly used in lab • What happens to medium when organisms are growing in it? How does it change over time? • continually altered by metabolic activities of organisms growing in it; nutrients depleted; wastes build up

Continuous culture•Fresh medium constantly added, used medium constantly removed, nutrient concentration stays same

Continuous culture•Fresh medium constantly added, used medium constantly removed, nutrient concentration stays same •Chemostat- continuous culture device; allows cell populations to remain in exponential growth for long periods

Last, First

• In last column of “name” field on back of scantron, bubble in your section number. • Leave the next to last column in “name” field blank.

blank lab section

LSU ID#

Do not touch

Section

Lab time

Letter to bubble in

1

7:40

A

2

9:10

B

3

10:40

C

4

12:10

D

5

1:40

E

6

3:00

F

7

4:30

G

Cell Differentiation Cells respond to changing environment • Endospores – Form inside (“endo”) mother cell

• Dormant survival structure formed by some species of Gram + rod-shaped bacteria during harsh conditions. • Ex. Bacillus & Clostridium • Resistant to heat, radiation, drying, acids, etc. • Can survive indefinitely. Vegetative cell

Sporosarcina endospore

• Sporulation- formation of an endospore when enviromental conditions are not favorable. • Germination- formation of a vegetative cell from an endospore when conditions are favorable.

sporulation

vegetative growth nutrient starvation

germination

sporulation free endospore

sporangium with endospore

sporangium is degraded

Endospore intracellular location Terminal Subterminal Central

Endospore inside cell

Structure of endospores

• Core- center of endospore. Contains cell wall, CM, cytoplasm & nucleoid. • Cortex- surrounds core. Made of loosely cross-linked peptidoglycan. • Spore coat- protein which covers cortex. • Exosporium- thin layer of protein which covers the spore coat

• Calcium–diplicolinic acid helps dehydrate endospore, stabilizes DNA & protects it from heat denaturation. • Small acid-soluble proteins protect DNA from UV radiation, desiccation, dry heat & also serve as carbon & energy source during germination.

Cell Differentiation Heterocysts  Different

cells produce different nutrients

Cell Differentiation Heterocysts  Different

cells produce different nutrients Vegetative cells— energy Heterocysts—fix N 2

Myxospores  Form

inside fruiting body Multicellular structure

– Actinomycetes form spores • Food runs out • Produce aerial hyphae • Disseminates cells

Streptomyces

Chapter 5 Environmental Influences and Control of Microbial Growth

Environmental factors that affect microbial growth • • • • •

Temperature Pressure Osmolarity pH Oxygen

Temperature • Temperature is a major environmental factor controlling microbial growth. • cardinal temperatures- minimum, optimum, & maximum temperatures for an organism • minimum temperature - cellular processes slow; cytoplasmic membranes stiffen • maximum temperature- proteins start to denature • optimum temperature- organism grows best; between min & max

•Microorganisms can be grouped by the temperature ranges they require.

• Psychrophiles –Cold: O°C–20°C • Mesophiles –20°C–45°C • Thermophiles –40°C–80°C • Extreme thermophiles –65°C–113°C

• Psychrophilesfound in constantly cold environments • Example: Chlamydomonas“snow algae”

pink snow algae Chlamydomonas

Molecular adaptations of psychrophiles: • Membranes have high content of unsaturated fatty acids - semi-fluid at low temperatures • Proteins are more flexible compared to mesophiles or thermophiles

• Cryoprotectants can be used to preserve microbial cultures at low temps • 10% DMSO (Dimethylsulfoxide) & 10% glycerol are commonly used in laboratories to preserve microbial cultures for long time in freezers.

• Mesophiles- midrange optimum temperature • Found in warm-blooded animals & many terrestrial & aquatic environments. • Examples- most organisms you are familiar with such as Escherichia coli (found in the human intestine).

• Thermophiles – Optimum temp above 40°C – Some Archaea have been found growing at temps above 110 °C Morning glory pool in Yellowstone

Places thermophiles are found: • soils subjected to full sunlight • fermenting materials (compost piles) • hot springs – Thermus aquaticus is a common hot spring thermophile. The heat stable DNA polymerase from this bacterium is mass produced and used in laboratories to replicate DNA in a test tube.

Grand prismatic spring in Yellowstone

• http://video.google.com/videoplay?docid=67

Molecular adaptations of thermophiles: • Membranes have a high content of saturated fatty acids – stable & functional at high temperatures • Enzymes are heat stable- proteins are more rigid compared to mesophiles or psychrophiles

• Heat shock response – Occurs at high end of temperature range – “Emergency” proteins produced – Help keep proteins from denaturing – Induced by many stressful conditions • Heat • High salt concentrations • Arid conditions

Pressure

• Barophiles – Adapted to high pressures • Up to 1,000 atm • Barotolerant organisms – Grow at high, but not very high pressure • Barosensitive organisms – Die at high pressure • Most “typical” bacteria, all mammals

Osmolarity

• Water moves from areas of high water concentration to areas of lower water concentration.

• Water moves from areas of low solute concentration to areas of high solute concentration.

• The diffusion of water is called osmosis.

H2O

In a hypotonic environment water will move into a cell.

H2O

In a hypertonic environment water will move out of a cell & the cell will die from plasmolysis.

• In a hypotonic environment the cell wall of most prokaryotes prevents too much water from entering cells even if equilibrium is never reached. •Isotonic – equal amount of solutes/water on inside & outside of cells

• However, there is no physical barrier that prevents cell from losing too much water if cell is in hypertonic environment. • Some cells can increase solute concentration in cell to prevent too much water loss by: 1. pumping inorganic ions (K+) into the cell; 2. making or concentrating an organic solute (glycerol) in the cell.

• Osmophile- organism that grows in high solute concentrations (hypertonic environments) • Halophiles-grow best in high salt habitats – Vibrio lives in ocean; % salt in ocean? • Extreme halophiles require high levels (15% to 30%) of salts for growth. – Halobacterium salinarium (requires 25% salt) lives in very salty lakes • Halotolerant- can survive at higher salt

concentrations but grow best in absence of salt

– Staphylococcus

Halobacterium salinarium

• End of exam 1 material! • The rest of Ch 5 will be covered on exam 2.

• Exam 2 material begins here • Chapter 5 continued

pH • pH- relative hydrogen ion concentration in a solution • Scale is from 0 to 14 • 7 is neutral, < 7 is acidic, > 7 is basic • Most bacteria grow at pH of 6-8 • Bacteria can be found to exist at almost any pH • Most cells internal pH remains near 7 regardless of pH of their environment

Sulfur oxidizing Bacteria and Archaea Iron oxidizing Bacteria Acetic acid Bacteria Lactic acid Bacteria Human intestinal flora cyanobacteria

Archaea extreme halophiles

• Most organisms have a pH range at which they can grow of 2-3 pH units. • Acidity or alkalinity of an environment can greatly affect microbial growth • Weak acids can pass through membranes – Good food preservatives

Classification based on optimal pH • Some organisms have evolved to grow best at low or high pH, but most organisms grow best between pH 6 & 8 & are called neutralophiles (neutrophiles).

Acidophiles- grow best at low pH • Stability of CM is critical since increases in pH can cause lysis • Ex. Many fungi, Thiobacillus produces sulfuric acid, volcanic thermal soil archaea Picrophilus oshimae grows optimally at pH 0.7

Sulfolobus- thermophilic and acidophilic

The pH Scale

0

7

Most bacteria and protozoa Most fungi

14

Alkaliphiles- grow best at high pH • found in soda lakes & high carbonate soil • Many species of Bacillus live in very alkaline soils • Bacillus firmus has a pH range of 7.5-11. • Proteases & lipases made by alkaliphiles are mass produced & used in household detergents. AKA: Alkalophiles Alkalinophile, Basophile

cyanobacterium Spirulina- alkaliphile

Oxygen • Microorganisms vary in their need or tolerance of oxygen (O2) & can be grouped based on their requirements for O2. • oxic environment- O2 is present • anoxic environment- no O2 is present

Aerobes- use O2 to generate energy by respiration • Facultative aerobes use O2 in respiration but can also grow in anoxic environments. Ex. Streptococcus mutans on teeth E. coli in large intestine • Obligate aerobe- use O2 in respiration & require oxic environments for growth. Grow at atmospheric O2 levels (21%). Ex. Micrococcus luteus

• Microaerophile- use O2 in respiration but require low O2 concentrations, 2-10%, (microoxic environments) to grow. Ex. Streptococcus pneumoniae

Anaerobes- cannot use O2 in respiration & may be inhibited or killed by O2. • Aerotolerant anaerobes- do not use O2 to generate energy but can survive in presence of it. – Ex. Streptococcus pyogenes • Obligate anaerobes- can only grow in anoxic environments; may die if even minute amount of O2 is present. – Ex. Clostridium sporogenes, Bacteroides in large intestine

• A reducing agent such as thioglycolate can be added to a medium to test an organism's requirement for O2. • Thioglycolate reacts with O2, reducing it to water. • In a culture medium, thioglycolate will convert all O2 to water; only top of culture is exposed to O2 in the air.

oa er op hil es Ae r An otol ae era ro b e nt s

Mi cr

Fa c Ae ulta ro tive be s

es An ae ro b

Ob Ae liga ro te be s

The position of the bacteria within these thioglycolate broth cultures reveals the O2 requirements for each of the bacteria.

• Special techniques are needed to grow aerobic & anaerobic microorganisms in the laboratory. • Aerobes• culture medium must be oxygenated by shaking or bubbling air into the medium.

Anaerobes need O2 to be excluded. • Bottles or tubes can be filled completely with media & sealed with a screw cap. • Reducing agents (thioglycolate) can be added to convert all O2 to water. • Anoxic jars with a palladium catalyst convert O2 to water.

• For obligate anaerobes that die if exposed to O2, media must be boiled, a reducing agent added, then sealed under an O2-free H2 or N2 gas. • Work with these cultures must be done in an anoxic environment that can be provided by anoxic glove boxes.

Toxic Forms of Oxygen

• Several toxic forms of oxygen or molecules that contain oxygen can be formed in the cell during normal cellular processes: Singlet oxygen 1O2 produced by peroxidases Superoxide anion O2Hydrogen peroxide H2O2 Hydroxyl radical -OH

generated during the reduction of oxygen to water

Enzymes made by cells can neutralize toxic forms of oxygen. Catalase, peroxidase, superoxide dismutase, superoxide reductase

These reactions generate reactive oxygen species (toxic oxygen products)

These reactions destroy ROS

Controlling Microbial Growth Physical Agents—Temperature • Pasteurization- 63°C for 30 minutes • Flash pasteurization- 72°C for 15 seconds • does NOT kill all cells • reduces microbial load (# of viable organisms) • kills most pathogens; inhibits spoilage microbes • UHT—Ultra-high temperature – 150°C for 3 seconds • Sterilizes—all bacteria killed – creamer, boxed milk

Physical Agents —Temperature + Pressure • Autoclave – 121°C, 15 psi, 20 min – Kills all bacteria – Destroys endospores

Physical Agents—Other Methods • Cold temperature – Refrigeration – Freezing • Slows growth, does not kill all bacteria

Physical Agents—Other Methods Irradiation • Microwaves – thermal effects • Ultraviolet radiation – DNA damage Ionizing radiation

X-rays gamma rays

nucleic acid and protein damage

Ultraviolet radiation • used to decontaminate surfaces & materials that do not absorb light (air & water) •Causes thymine dimers in DNA.

•UV hood – air is blown outward through a filter from the back and from edges of the hood so that the area inside the hood remains sterile once the UV light is turned off.

Ionizing radiation • Gamma rays & X-rays • penetrates solid or light-absorbing materials • widely used for sterilization & decontamination (treatment of an object or surface to make it safe to handle) in medical & food industries • Causes breaks in DNA; breaks hydrogen bonds & disulfide bridges in proteins

Physical Agents—Other Methods

Filtration  Filter-device with pores too small for microorganisms to fit through but large enough for liquid or gas to pass through.

Physical Agents—Other Methods

Filtration  Filter-device with pores too small for microorganisms to fit through but large enough for liquid or gas to pass through.  Filters remove microorganisms from air or liquids that are heat sensitive.  2 types: depth & membrane

Depth filters • fibrous sheets or mats made from a random array of overlapping paper, asbestos, or borosilicate • traps large particles from liquids & air Examples • HEPA filters • Home air/heat system • Vacuum cleaner • UV hood • Clean rooms and isolation rooms for quarantine

Membrane filters • thin sheets of polymers (cellulose); contain tiny holes of known size • Act like sieves, trap particles on membrane surface • Antibiotics & other pharmaceuticals • Nucleation track (Nucleopore) filters used for concentrating a liquid sample for view on the scanning electron microscope.

Chemical Agents

• Disinfectants – used to reduce microbial numbers on nonliving material • bleach (chlorine), ethanol • Antiseptics – used to reduce microbial numbers on living tissues • Betadyne (iodine), H2O2

Chemical Agents

Antibiotics • naturally occurring antimicrobial substances produced by microorganisms • Many known but less than 1 % clinically useful because of poor uptake or toxicity. • Selectively kills microbes – May not work on all species • Interferes with bacterial-specific enzymes – Cell wall synthesis – Bacterial ribosome

Penicillin

• Many derivatives • Blocks cell wall synthesis • Growing bacteria lyse – Slow-growing bacteria take longer to die

Biological Agents

• Probiotics – “Good” bacteria • Displace pathogens from tissues • Bacteriophage – “Phage” – Viruses that infect bacteria • Do not harm eukaryotes