“ … 251, 252, 253, 254 … wait, where was I? Okay, 1, 2, 3 … ”
Counting may be as easy as 1, 2, 3, but when you’re keeping track of microscopic organisms, it can be a real headache (literally).
Getting an accurate count of these microorganisms is often incredibly important. By counting the colonies or Colony Forming Units (CFUs) in a diluted sample, a scientist can estimate the number of bacterial or fungal cells in the original sample. “This is a vital piece of data in a range of applications, including human and animal infection diagnostics, food safety as well as general microorganism research, such as antibiotic studies,” explains Robert Skehens, marketing director at Bibby Scientific. “The challenge is that the colonies can be very small and are usually opaque, making them difficult to count.”
“The most frustrating part is that counting colonies is quite boring and lowbrow,” says Silvio Brugger, a senior physician in internal medicine at University Hospital Bern in Switzerland, who has published on automated colony counters [1]. “Nevertheless, it needs a certain level of concentration, and therefore it is fatiguing.” Once upon a time, you’d have had no choice but to stare at your petri dish, hoping to spot every colony and not lose count. But today there are machines that can help, relieving the tedium and improving the accuracy of your colony count.
Simple to sophisticated
Colony counters come in several basic categories, with increasing levels of assistance, automation and price. The cheapest option is essentially a touch-sensitive pen. It simply records each time you mark a colony with the pen on the lid of your dish, with a number display ticking up with each press. It’s cheap and portable, but it won’t offer any backlighting to help make your colonies more visible, and you’ll want to have replacement cartridges handy for when the pen runs out of ink.
Cole-Parmer offers a very basic Digital Counter Pen™ with an LCD display for about $20 (replacement cartridges are $7.25 each). Scienceware has a more sophisticated handheld instrument, the Scienceware Colony Counter, which holds a special marker pen and can store up to 35 sample counts. It costs about $230, with replacement pens costing $8.
The step up from a pen counter is a ‘manual’ colony counter, which you can usually get for less than $1,000. Pen counters provide an illuminated area on which to rest your dish, and each time you mark a colony with a felt-tip pen (any can be used) the pressure advances the counter , often with an audible beep. These instruments can average your counts but won’t retain any information (although you may be able to connect them to a computer to upload your data).
Bibby Scientific offers the SC6Plus manual colony counter for £495 ($835). This model features all of the above plus computer connectivity; a choice of magnifiers and a printer are offered as options. (The company plans to add digital image capture to the machine, so a visual record of the plate can be kept for future reference, says Skehens). The aCOLade from Synbiosis, a division of the Synoptics group, offers similar features at roughly the same price.
Automation
After you start looking at fully automated options, the features increase, but so does the cost. You could easily be looking at $5,000 or even $20,000 for something particularly high spec. Instruments at this level come with a digital camera that takes a picture of the plate that can be saved for performed manually. “If you work in a lab where a lot of plates have to be counted, like hygiene control, it will save you a lot of time,” says Brugger, so it’s a case of weighing the financial investment vs. the time and effort of colony counting in your specific setting.
“Automatic counters enable high-throughput counting of hundreds or thousands of plates,” explains Jayne Arthur, marketing communications manager at Synoptics. “They are fast, there are no transcribing errors, and there’s a data trail of who performed the counting and when.” Synbiosis’ aCOLyte 3 fully automatic imaging light box for colony counting pour plates and spiral plates is on the market for £2,500 ($4,200); it incorporates a high-res digital camera and live, digital video link via USB. Plates are automatically counted in just one second, with results directly transferred to Excel or Open Office, and colonies down to 0.3 mm can be imaged and counted. UVP’s ColonyDoc-It™ offers similar features, including a choice of light source, but is able to capture colony sizes as small as 0.08 mm. It’s on the market for about $7,000.
Falling digital-camera costs are beginning to bring down the price of these automated systems, but it’s not all about the money. “The main issue with a digital camera and software—deciding if a tiny shape on a dish is indeed a colony—is it’s very difficult for the traditional microbiologist to accept,” say Bibby Scientific’s Skehens. “Most end up doing a manual count anyway, trusting their own eye and judgment above all else.”
Top of the range
A step up from even these products is something like ProtoCOL3, which is an automated colony-counting and zone-sizing system. But at £12,000 ($20,200), it’s only an option for those who can justify the financial outlay. This particular model will provide data on the size and shape of your spots, count colonies down to 0.043 mm and measure zones of inhibition to 0.1 mm. It can count and calculate concentrations of spiral-plated colonies or quantify Ames test plates. Advanced systems like this can read barcode labels, calculate CFUs and transfer data to your computer to provide a regulatory audit trail.
Interscience offers three levels of automated colony counter in its Scan range, from the “budget friendly” Scan® 300 ($7,550 from Thomas Scientific) to the “high resolution” Scan 1200 ($17,900). The latter can measure down to 0.05 mm, differentiate colonies by color and handle Petrifilm™, RIDA™ Count/Sanita-kun™, Compact Dry™ and filtration membranes. But these extra features place this model near the top of the price list.
In addition to thinking about your budget and desired level of automation, there are small practicalities to remember. Does the model you’re considering accept the size of plate you work with, for example? How large is the machine itself? Does the software come in your language?
Another consideration: how much bench space will the unit require? One counter actually fits in your pocket, and costs less than your morning Starbucks. The Promega Colony Counter app is available for iPhone and iPad for $3.99, and it will give you an automated count of your colonies based on a photo of the plate taken using the app. You can then refine the count manually, zooming in on one quadrant at a time, and average your count over several plates, saving the data for later.
Whichever option you choose, colony counters can make light work of what would otherwise be a very labor-intensive and tedious—albeit vital—task. Choosing the right model can help make a lab more efficient and cure at least one headache for the busy scientist.
Reference
[1] Brugger, S.D., et al., “Automated counting of bacterial colony forming units on agar plates,” PLoS ONE, 7:e33695, 2012. [PubMed ID: 22448267]