Science 7–10 · Year 9

Microorganisms on hands: sealed agar plates before and after hand hygiene

Science understanding: Biological sciences (NSW Stage 5 focus area Disease; no ACARA v9 Science understanding content description deals with microorganisms on the skin or hand hygiene, so the ACARA side is coded to Year 9 science inquiry only)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Hands carry living microorganisms that grow into visible colonies on a nutrient surface, and hand hygiene reduces how many are transferred without sterilising the skin.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Unknown microorganisms may grow, including potential pathogens if plates are opened or incubated warm

Controls

Sample hands only, never body fluids, toilets or animals (RiskAssess rules)
Plates taped and never opened after growth; counts through the lid
Incubation at or below 30 °C
All plates sterilised in an autoclave or pressure cooker before disposal
Hands washed after handling plates

Note

Culturing follows the RiskAssess "Issues relating to culturing of microorganisms" rules for Australian schools: sample only hands or the environment, tape plates with two to four short strips, incubate at or below 30 °C, never open a plate after growth, and sterilise plates in an autoclave or pressure cooker before disposal. Record a RiskAssess risk assessment.

What you need

Nutrient agar plates poured and set by the technician, two per learner (four per pair), kept lidded and inverted
Marker pen, clear adhesive tape
Soap, running water, paper towel, and an alcohol hand rub for a third condition
An incubator or a cupboard held at or below 30 °C (RiskAssess: do not incubate above 30 °C)
Autoclave or pressure cooker and autoclave bags for disposal (technician)

How to do it

Each learner uses two plates. Label the base of each with initials, date, hand and condition. In each pair one learner tests a soap wash and the other an alcohol hand rub, and across the class half the learners leave the left hand unwashed and half the right.
Before washing, press the four fingertips of the unwashed hand lightly on the agar of the unwashed plate for five seconds; close the lid.
Then treat both hands, either washing with soap and water using the WHO technique (40 to 60 seconds for the whole procedure) and drying with paper towel, or rubbing in alcohol hand rub until dry. Press the four fingertips of the other hand, which has not yet touched agar, on the treated plate for five seconds.
Tape each lid to its base with two to four short strips (not around the whole rim), invert, and incubate at 25 to 30 °C for two to three days.
Count colonies through the closed lid on each plate and record colony size and colour; never open a plate.
Hand plates to the technician for autoclaving.

What you should see

Colonies become visible within 24 to 72 h at 25 to 30 °C. The unwashed plate is expected to carry the most colonies, of mixed sizes and colours, and the washed and hand-rub plates fewer. Washing does not sterilise the skin, so the washed plates still grow some colonies. The learner knows it worked when the unwashed plate shows countable separate colonies and the pooled class counts show fewer colonies after each treatment than on the unwashed plates.

What changes

What you change: Hand treatment (none, WHO soap wash, alcohol hand rub)
What you measure: Number of colonies per plate
What you keep the same: Same person for the unwashed and treated plates
Each hand pressed on agar once only
Four fingertips, pressing time and pressure
Hand left unwashed balanced across the class
Incubation temperature and time
Plate batch

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Clean-looking hands carry no microorganisms.
A quick rinse removes microbes as well as a full soap wash.
All the colonies are harmful; most are harmless skin residents.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-DIS-01 SC5-WS-03 SC5-WS-04 SC5-WS-06
Australian Curriculum v9AC9S9I02AC9S9I03AC9S9I05

Sources

The pages the author read to write this activity.

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