C6 Brock Sample Questions

Question 1

Which of the following is/are transferred to daughter cells during bacterial cell division?
A) only genomic DNA, the blueprint for everything the cell needs
B) individual genes and enzymes to later be brought together to form the genome
C) only the chromosome and enzymes to immediately begin cellular metabolism
D) chromosome, proteins, and all other cellular constituents

Answer 1

D) chromosome, proteins, and all other cellular constituents

During bacterial cell division (binary fission), the entire cellular content, including the chromosome, proteins, and other cellular constituents, is replicated and divided between the two daughter cells.

Question 2

Bacterial growth refers to
A) an increase in the number of cells.
B) the occurrence of binary fission.
C) the time required for one cell to divide into two.
D) the occurrence of mitosis.

Answer 2

A) an increase in the number of cells.

Bacterial growth is defined as an increase in the number of bacterial cells, which can be measured through various methods such as cell counting, turbidity, or colony formation.

Question 3

The time interval required for the formation of two cells from one is called the A) generation time.
B) growth time.
C) growth rate.
D) division rate.

Answer 3

A) generation time.

Generation time, also known as doubling time, is the time required for a bacterial cell to divide into two daughter cells, effectively doubling the population.

Question 4

Turbidity measurements are commonly utilized for monitoring
A) planktonic cultures.
B) biofilms.
C) conies.
D) sessile cultures.

Answer 4

A) planktonic cultures.

Turbidity measurements are often used to monitor the growth of planktonic cultures (suspended cells in a liquid medium), as increased cell density correlates with increased turbidity.

Question 5

The time between inoculation and the beginning of growth is usually called the
A) lag phase.
B) log phase.
C) dormant phase.
D) death phase.

Answer 5

A) lag phase.

The lag phase is the initial period after inoculation where bacteria adapt to their new environment, during which there is little to no noticeable growth.

Question 6

Optical density and viable cell concentration are LEAST proportional to each other during the
A) lag phase.
B) exponential growth phase.
C) stationary phase.
D) death phase.

Answer 6

D) death phase.

In the death phase, cells are lysing or dying, which can lead to a decrease in viable cell concentration without a corresponding decrease in optical density (as dead cells and debris still contribute to turbidity).

Question 7

When attempting to determine viable cell counts of a heat-sensitive bacterial isolate, which cell counting method should be avoided?
A) spread-plate method
B) pour-plate method
C) live staining and direct counting with a microscope
D) both spread- and pour-plate methods

Answer 7

D) both spread- and pour-plate methods (assuming both involve heat; otherwise, pour-plate method is more likely to involve heat)

Heat-sensitive isolates may be killed or inactivated by the heat used in pour-plate methods (melting agar) or potentially in spread-plate methods if the agar is too warm.

Question 8

Bacteria that are able to grow in humans and cause disease would be classified as
A) psychrophiles.
B) mesophiles.
C) thermophiles.
D) hyperthermophiles.

Answer 8

B) mesophiles.

Mesophiles are organisms that grow best in moderate temperatures (20-45°C), which is consistent with the human body temperature (37°C), making them capable of causing disease in humans.

Question 9

An agar plate for counting colonies and maximizing statistical validity should ideally contain
A) 1 to 100 colonies.
B) 50 to 100 colonies.
C) 30 to 300 colonies.
D) 100 to 1000 colonies.

Answer 9

C) 30 to 300 colonies.

This range is often recommended for optimal statistical analysis, as it provides a sufficient number of colonies for accurate counting without being too numerous to count reliably.

Question 10

Relative to enzymes in mesophilic microorganisms, which of the following is NOT characteristic of enzymes in psychrophiles?
A) decreased alpha helix content
B) decreased beta sheet content
C) fewer hydrogen bonds
D) fewer ionic bonds

Answer 10

D) fewer ionic bonds

Psychrophile enzymes are adapted for activity at low temperatures, often involving increased flexibility, which might not directly relate to “fewer ionic bonds” as a primary characteristic.

Question 11

The number of colonies obtained in a plate count does NOT depend on the A) inoculum volume.
B) size of the colonies.
C) type of culture medium.
D) incubation time.

Answer 11

B) size of the colonies.

The number of colonies in a plate count primarily depends on the initial inoculum volume, type of culture medium, and incubation time. Colony size can vary based on growth conditions and bacterial type but doesn’t directly influence the initial colony count.

Question 12

Cell density in a chemostat is controlled by
A) the concentration of the limiting nutrient.
B) the dilution factor.
C) the size of the initial inoculum.
D) the concentration of the limiting nutrient, the dilution factor, and the size of the initial inoculum.

Answer 12

D) the concentration of the limiting nutrient, the dilution factor, and the size of the initial inoculum.

In a chemostat, cell density is controlled by the concentration of the limiting nutrient (affecting growth rate), the dilution factor (affecting population washout rate), and to some extent, the initial inoculum size (initial condition).

Question 13

What temperature is most commonly used in autoclaves to sterilize growth media and other devices prior to experimentation?
A) 95°C
B) 101°C
C) 121°C
D) 140°C

Answer 13

C) 121°C

Autoclaving typically involves steam sterilization at 121°C (250°F) for 15-20 minutes to achieve effective sterilization.

Question 14

Most mesophilic organisms can grow in a temperature range of
A) 0-15°C.
B) 10-20°C.
C) 20-45°C.
D) 50-65°C.

Answer 14

C) 20-45°C.

Mesophiles are defined by their growth in moderate temperatures, which typically range from about 20°C to 45°C, with an optimal temperature often around 37°C.

Question 15

Obligate anaerobes which are sensitive to O2 would be found growing
A) throughout a tube of thioglycolate broth.
B) only at the very top of a tube of thioglycolate broth.
C) only at the bottom of a tube of thioglycolate broth.
D) approximately one-third of the way down the thioglycolate broth.

Answer 15

C) only at the bottom of a tube of thioglycolate broth.

Obligate anaerobes cannot survive in the presence of oxygen. In a thioglycolate broth tube, the bottom would be the most anaerobic environment, allowing these bacteria to grow.

Question 16

A microbe growing in a refrigerator is likely
A) psychrophilic.
B) mesophilic.
C) psychrotolerant or psychrophilic.
D) hyperthermophilic.

Answer 16

C) psychrotolerant or psychrophilic.

Growth in a refrigerator (around 4°C) suggests the microbe is either psychrotolerant (can grow in cold but has a higher optimal temperature) or psychrophilic (has an optimal growth temperature in the cold).

Question 17

Which of these statements is/are TRUE?
A) In general, species that can grow at higher temperatures are prokaryotic.
B) The most thermophilic prokaryotes are species of Archaea.
C) Chemoorganotrophic organisms are able to grow at higher temperatures than phototrophic organisms.
D) All of the statements are true.

Answer 17

All of the statements are true.

Without the specific statements, this answer assumes all provided statements about temperature, prokaryotes, and growth capabilities are generally accurate.

Question 18

A chemical that denatures proteins is most likely to be classified as a(n) agent.
A) antiseptic
B) bacteriostatic
C) bactericidal
D) detergent

Answer 18

D) detergent

Detergents are known to denature proteins by disrupting their structure, making this the most fitting classification among the options provided.

Question 19

The ratio of the vapor pressure of the air in equilibrium with a substance to the vapor pressure of pure water is known as
A) water activity.
B) vapor activity.
C) positive water balance.
D) osmosis.

Answer 19

A) water activity.

Water activity (aw) is a measure of the energy status of water in a system, which directly relates to the vapor pressure of the substance compared to pure water.

Question 20

Organisms able to live in environments with high sugar concentrations are A) halotolerant.
B) osmophiles.
C) xerophiles.
D) anaerobic fermenting bacteria.

Answer 20

B) osmophiles.

Osmophiles are organisms that thrive in high solute concentrations, such as high sugar environments, where the osmotic pressure is elevated.

Question 21

When water activity is low, an organism must
A) increase its internal solute concentration.
B) increase its external solute concentration.
C) decrease its internal solute concentration.
D) decrease its external solute concentration.

Answer 21

A) increase its internal solute concentration.

To survive in low water activity environments (high osmotic pressure), organisms often increase their internal solute concentration to maintain cellular integrity and function.

Question 22

A bacterium possessing ______ will produce O2 bubbles when placed in the presence of hydrogen peroxide.
A) catalase
B) superoxide dismutase
C) superoxide reductase
D) peroxidase

Answer 22

A) catalase

Catalase is an enzyme that decomposes hydrogen peroxide into water and oxygen, which would produce O2 bubbles.

Question 23

A halotolerant facultative anaerobe would grow BEST in which of the following environments?
A) oxygen depleted saline
B) oxygenated saline
C) oxygen depleted non-saline
D) oxygenated non-saline

Answer 23

B) oxygenated saline

As a halotolerant organism, it can grow in saline environments. Being a facultative anaerobe, it can grow with or without oxygen, but “best” growth often occurs in the presence of oxygen when available.

Question 24

Which of the following forms of oxygen is/are generally toxic to living organisms?
A) superoxide anion
B) hydrogen peroxide
C) hydroxyl radical
D) Superoxide, hydrogen peroxide, and hydroxyl radicals are all toxic.

Answer 24

D) Superoxide, hydrogen peroxide, and hydroxyl radicals are all toxic.

All the mentioned forms of reactive oxygen species (ROS) are known to be toxic to living organisms, causing oxidative stress and damage.

Question 25

Superoxide dismutase and catalase work together to convert superoxide into
A) peroxide.
B) oxygen.
C) water
D) water and oxygen.

Answer 25

D) water and oxygen.

Superoxide dismutase converts superoxide into hydrogen peroxide, which is then converted into water and oxygen by catalase, providing a complete detoxification pathway.

Question 26

Which of the following methods used to enumerate cells often requires specialized staining to observe non-pigmented bacteria?
A) spectrophotometry / turbidity
B) spread-plating
C) microscopy
D) spread-plating, turbidity, and microscopy

Answer 26

C) microscopy

Microscopy, especially for non-pigmented (colorless) bacteria, often requires specialized staining techniques to visualize the cells.

Question 27

Which term is most relevant in describing the efficacy of an antimicrobial for use in a clinical setting? A) effective dose
B) sterilization coefficient
C) lethal dosage
D) minimum inhibitory concentration (MIC)

Answer 27

D) minimum inhibitory concentration (MIC)

The MIC is a measure of the lowest concentration of an antimicrobial that can inhibit the growth of a microorganism, making it highly relevant for clinical efficacy.

Question 28

Which method would be LEAST effective at sterilizing a glass hockey stick to use in the spread-plate method?
A) autoclaving
B) gamma radiation
C) ethanol soaking
D) ultrahigh-temperature pasteurization

Answer 28

C) ethanol soaking

Ethanol soaking, while effective against many vegetative cells, may not be sufficient to sterilize all forms of microbial life, especially spores, on a surface like a glass hockey stick.

Question 29

When counting colonies on an agar plate
A) it is assumed that each colony arose from division of one or a few cells.
B) only viable colonies are counted.
C) the medium must be suitable for colony growth.
D) All of these answer choices are true.

Answer 29

D) All of these answer choices are true.

Question 30

By controlling the concentration of nutrients continuously added to a chemostat, cells can constantly be maintained at the
A) exponential growth phase.
B) stationary growth phase.
C) lag growth phase.
D) death growth phase.

Answer 30

A) exponential growth phase.

A chemostat allows for the continuous addition of nutrients and removal of waste, maintaining a steady state that can keep cells in a continuous exponential growth phase.

Question 31

Serial dilutions are generally made
A) when the appropriate viable count is unknown.
B) based on powers of 10, although other serial dilution factors are possible.
C) to reach a suitable dilution while minimizing error.
D) All of these answer choices apply.

Answer 31

D) All of these answer choices apply.

Question 32

Which of the following statements is FALSE?
A) In the death phase, bacterial growth may cease as a result of oxygen and nutrient depletion.
B) The generation time of bacteria may vary from species to species.
C) Bacteria typically divide by binary fission, producing two daughter cells.
D) In the lag phase, cell death exceeds cell division.

Answer 32

D) In the lag phase, cell death exceeds cell division.

During the lag phase, cells are adapting to their environment; while there might be some cell death, the characteristic of this phase is not that cell death exceeds cell division, but rather that there’s a significant increase in cellular machinery preparation for division.

Question 33

The optimal growth temperature of a bacterium is most closely related to the optimal temperature for
A) transcription of DNA.
B) DNA replication.
C) enzyme function.
D) mRNA attachment to ribosomes.

Answer 33

C) enzyme function.

Enzyme function and stability are crucial for bacterial metabolism and growth. The optimal growth temperature often reflects the temperature at which the bacterial enzymes are most active and stable.

Question 34

Why is the enzyme catalase important to the survival of bacteria?
A) Catalase protects the cell from desiccation.
B) Catalase breaks down toxic hydrogen peroxide.
C) Catalase allows for growth in high salt concentrations.
D) Catalase aids in the transport of oxygen into the cell.

Answer 34

B) Catalase breaks down toxic hydrogen peroxide.

Catalase protects bacterial cells from oxidative damage by converting hydrogen peroxide (H2O2), a harmful reactive oxygen species, into water and oxygen.

Question 35

A medium containing known substances in precise amounts is known as a media.
A) selective
B) complex
C) natural
D) defined

Answer 35

D) defined media.

Defined media are formulated with precisely known quantities of components, allowing for the growth of organisms with specific nutritional requirements and facilitating reproducible experimental conditions.

Question 36

Selective medium differs from differential medium because
A) selective medium permits growth of more organisms than differential medium.
B) selective medium permits growth of a particular microbial type while differential medium is used to distinguish between types of organisms.
C) differential medium differentiates pathogens from nonpathogens while selective medium grows only pathogens.
D) differential medium contains growth inhibitors while selective medium does not.

Answer 36

B) selective medium permits growth of a particular microbial type while differential medium is used to distinguish between types of organisms.

Selective media inhibit the growth of certain microorganisms while allowing others to grow, based on specific conditions or additives. Differential media, however, allow for the growth of multiple types of organisms but provide a way to distinguish between them based on observable characteristics.

Question 37

The cell membranes of thermophiles and hyperthermophilic bacteria typically have
A) more saturated fatty acids.
B) more long chain fatty acids.
C) fewer unsaturated fatty acids.
D) All of these answer choices are true of thermophiles and hyperthermophilic bacteria.

Answer 37

D) All of these answer choices are true of thermophiles and hyperthermophilic bacteria.

Question 38

The use of chemical agents directly on exposed body surfaces to destroy or inhibit vegetative pathogens is
A) disinfection.
B) sterilization.
C) antisepsis.
D) sanitization.

Answer 38

C) antisepsis.

Antisepsis involves the application of antimicrobial substances to living tissues, such as skin or mucous membranes, to reduce or eliminate microbial growth.

Question 39

Which of the following statements is TRUE of disinfectants?
A) They are always effective in destroying endospores.
B) They are used on living tissue.
C) They are used for sterilization.
D) They are used on inanimate surfaces.

Answer 39

D) They are used on inanimate surfaces.

Disinfectants are chemical agents designed to inactivate or destroy microorganisms on non-living surfaces, distinguishing them from antiseptics used on living tissues.

Question 40

A _____ agent is a chemical that inhibits bacteria from reproducing, but does NOT necessarily kill them.
A) bacteriostatic
B) bacteriolytic
C) bactericidal
D) xerophilic

Answer 40

A) bacteriostatic

Bacteriostatic agents slow down or halt bacterial growth and reproduction without necessarily killing the bacteria, as opposed to bactericidal agents which kill bacteria.

Question 41

Microbial contamination is prevented by the
A) use of the quadrant streak method.
B) use of aseptic technique.
C) use of cultures containing special nutrients.
D) spread plate method.

Answer 41

B) use of aseptic technique.

Aseptic technique involves procedures and practices that minimize the introduction of microorganisms into an environment, effectively preventing contamination.

Question 42

A common antimicrobial chemical (or chemicals) used as both an antiseptic and a general disinfectant is
A) iodophors.
B) 70% alcohol.
C) hydrogen peroxide.
D) Each of these chemicals at appropriate concentrations can be used as antiseptics or disinfectants.

Answer 42

D) Each of these chemicals at appropriate concentrations can be used as antiseptics or disinfectants.

Depending on concentration and application, various chemicals (e.g., iodophors, 70% alcohol, hydrogen peroxide) can serve as either antiseptics (on living tissue) or disinfectants (on non-living surfaces).

Question 43

Cultures of a bacterial species were incubated on the shelf of a refrigerator (5°C), on a lab benchtop (25°C), on the shelf of a 37°C incubator, and on the shelf of a 50°C incubator. After incubation, there was no growth at 37°C or 50°C, slight growth on the benchtop, and abundant growth at refrigeration. What term could be used for this species?
A) halophile
B) mesophile
C) anaerobe
D) psychrophile

Answer 43

D) psychrophile

Given the abundant growth at refrigeration temperatures (5°C) and no growth at higher temperatures (37°C, 50°C), this organism thrives in cold conditions, characteristic of psychrophiles.

Question 44

A microbiology student in a research lab repeated an experiment described by others using the identical bacterial isolate, the same growth medium, and the same growth conditions. However, the student was unable to achieve the same O.D. at 600 nm. What is the LEAST likely cause for this discrepancy in measured turbidity?
A) One experiment used 16 mm wide test tubes while the other used 18 mm wide test tubes.
B) One experiment subtracted the yellow color of the growth medium away from the final turbidity reported whereas the other used colorless water.
C) One experiment vigorously dispersed the bacterial pellicle with vortexing while the other did not.
D) The two experiments were carried out at vastly different elevations and did not consider the influence of pressure.

Answer 44

D) The two experiments were carried out at vastly different elevations and did not consider the influence of pressure.

While several factors could influence turbidity measurements, the effect of vastly different elevations (and thus pressure) on the turbidity of bacterial cultures is likely to be minimal compared to the other options provided (e.g., tube size, medium color correction, and sample mixing).

Question 45

Microbial growth is generally described as an increase in cell number rather than the expansion in size of an individual microbial cell.

Answer 45

TRUE

Microbial growth is typically measured by an increase in cell number, not the size of individual cells.

Question 46

The duration of exponential growth would increase if bacterial cells divided into three equal daughter cells rather than two.

Answer 46

FALSE

Exponential growth is characterized by cells dividing into two equal daughter cells, not three.

Question 47

A selective medium is used to suppress the growth of certain bacteria in order to facilitate the growth of other bacteria.

Answer 47

TRUE

Selective media are designed to inhibit the growth of certain bacteria, allowing others to grow more prominently.

Question 48

The lag phase does NOT occur if an exponentially growing culture is transferred into a nutrient rich medium with the same components and growth conditions as the previous medium.

Answer 48

FALSE

The lag phase can still occur if the transfer conditions are significantly different, but if all conditions remain the same, the culture might continue growing exponentially without a lag phase.

Question 49

The rate of exponential growth varies greatly according to bacterial species and bioavailable nutrients.

Answer 49

TRUE

Both the bacterial species’ inherent growth rate and the availability of nutrients significantly influence the rate of exponential growth.

Question 50

A bacterium whose optimal growth temperature is 35˚C would be classified as a thermophile

Answer 50

FALSE

A thermophile typically has an optimal growth temperature above 50°C; 35°C is more characteristic of mesophiles.

Question 51

A complex medium is one that has been completely made in the laboratory and the concentration of all the nutrients is known.

Answer 51

FALSE

A complex medium is defined as one containing unknown or variable concentrations of nutrients, often including extracts or undefined components.

Question 52

Many fungi and bacteria grow best at pH 5 or below.

Answer 52

TRUE

Many microorganisms, especially fungi and some bacteria, have optimal growth at acidic pH levels, around or below pH 5.

Question 53

In both the lag and stationary phase, there is no net increase or decrease in viable cells.

Answer 53

FALSE

1. In the lag phase, there’s often an increase in viable cells as they adapt and prepare for growth.
2. In the stationary phase, there’s typically no net increase, but there can be a decrease in viable cells over time due to death and lysis.

Question 54

The death phase applies to individual cells rather than populations.

Answer 54

FALSE

The death phase is a population-level phenomenon, characterized by a significant decrease in the number of viable cells within the population.

Question 55

Direct microscopic counting of cells is an accurate method for estimating the number of viable cells in a sample.

Answer 55

TRUE

Microscopic counting can be highly accurate for estimating viable cell numbers, especially when combined with viability stains.

Question 56

The optimal pH for growth of an organism refers to its intracellular environment.

Answer 56

FALSE

The optimal pH for growth typically refers to the external environment’s pH, which supports the best growth of the organism, not necessarily the intracellular pH.

Question 57

When viable cell concentrations are too high to count on an agar plate, it is common to use larger sized plates to increase the surface area for counting the colonies.

Answer 57

FALSE

When concentrations are too high, it’s more common to dilute the sample (e.g., through serial dilutions) rather than use larger plates to facilitate counting.

Question 58

Refrigeration controls microbial growth in food as a result of irreversible cell damage caused by low temperature.

Answer 58

FALSE

Refrigeration primarily slows microbial growth by reducing metabolic rates, not necessarily causing irreversible cell damage.

Question 59

The optimum growth temperature for an organism is typically closer to its minimum temperature rather than the maximum temperature.

Answer 59

FALSE

The optimal growth temperature is often closer to the organism’s maximum preferred temperature, especially for thermophiles and psychrophiles, which have optimal growth temperatures near their extreme tolerance limits.

Question 60

A bacterium such as a snow alga that is able to grow in cold temperatures is called a psychrophile.

Answer 60

TRUE

Psychrophiles are indeed defined by their ability to grow in cold temperatures, with snow algae being a characteristic example.

Question 61

Some microbes can grow in boiling water.

Answer 61

TRUE

Certain microorganisms, especially thermophiles, can indeed grow in extremely hot environments, including boiling water.

Question 62

Organisms living in boiling hot springs often grow rapidly and have short doubling times.

Answer 62

TRUE

Thermophilic organisms in hot springs often exhibit rapid growth rates, contributing to short doubling times.

Question 63

Taq polymerase was isolated from a thermophile and is used in the polymerase chain reaction (PCR) technique because it does not become inactivated at high temperatures.

Answer 63

TRUE

Taq polymerase, derived from the thermophilic bacterium Thermus aquaticus, retains its activity at the high temperatures required for PCR, making it ideal for this application.

Question 64

Mesophiles typically have longer fatty acid tails and more saturated carbon-carbon bonds in their cytoplasmic tails compared with thermophiles.

Answer 64

FALSE

Thermophiles, not mesophiles, are more likely to have longer fatty acid tails and more saturated carbon-carbon bonds to maintain membrane stability at high temperatures.

Question 65

Knowledge of microbial growth patterns is useful in working with the control of microbial growth.

Answer 65

TRUE

Understanding microbial growth patterns (e.g., lag, log, stationary, death phases) is crucial for effectively controlling microbial growth in various applications.

Question 66

Irreversible cell damage is more likely to occur at low rather than high temperatures.

Answer 66

FALSE

High temperatures are more likely to cause irreversible cell damage due to denaturation of proteins, membrane fluidity changes, and other thermal disruptions.

Question 67

Based on studies with obligate acidophiles, high concentrations of hydrogen ions are required for their membrane stability.

Answer 67

TRUE

Obligate acidophiles require a low pH environment (high hydrogen ion concentration) for their survival and membrane stability, adapting to conditions that would be lethal to most other organisms.

Question 68

A bacterial cell in nutrient broth with a generation time of 15 minutes will produce 16 cells in 2 hours.

Answer 68

FALSE

Given a generation time of 15 minutes, the correct calculation for the number of cells after 2 hours (or 8 doubling periods) is 1 initial cell * 2^8 = 256 cells.

Question 69

Planktonic cells remain in suspension while sessile cells adhere to a surface.

Answer 69

TRUE

This statement accurately describes the two lifestyles: planktonic cells are free-floating, whereas sessile cells attach to surfaces.

Question 70

The thermal death time is the time needed to kill all the bacteria in a particular culture at a certain temperature.

Answer 70

TRUE

Thermal Death Time (TDT) indeed refers to the length of time required to kill a given population of bacteria at a specified temperature.

Question 71

Autoclaving and pasteurization are two processes that are both very effective for sterilization.

Answer 71

FALSE

Autoclaving is highly effective for sterilization (killing all forms of life). Pasteurization, however, is typically used for disinfection or reducing the number of pathogens to a safe level, not necessarily achieving complete sterilization.

Question 72

Most microorganisms are particularly susceptible to antimicrobial agents during the logarithmic growth phase.

Answer 72

TRUE

Many antimicrobial agents target processes that are more active during the logarithmic (or exponential) growth phase, such as cell wall synthesis and DNA replication, making microbes more susceptible during this phase.

Question 73

Sterility of a laminar flow hood is accomplished by filter-sterilized air passed through the hood quickly enough that non- sterile air does not flow into the work area.

Answer 73

TRUE

Laminar flow hoods maintain sterility by continuously passing filtered (sterilized) air across the work surface at a sufficient velocity to prevent the ingress of non-sterile air from the surrounding environment.