All this week鈥檚 questions and answers come from the Last Word website. These give only a taste of the answers and conversations that take place online. If you enjoy this weekly magazine column but would like more, or if you want an answer to a question quickly, visit 鈥� Ed
Virtually every antibacterial cleaning product I buy claims it will kill 99.9% of bugs. So my question is what type of bacteria or viruses are the 0.1% that these products can鈥檛 kill? They really must be the sort of thing that you wouldn鈥檛 want to meet down a dark alley.
鈥� This figure is derived from the standard tests used to measure the antibacterial effectiveness of a product. The tests include EN1276, a disinfectant and antiseptics suspension test in which known concentrations of bacteria are treated with a product for a given contact time, usually 5 minutes. You may start off with 5 million bugs per millilitre of test solution and, after treatment, end up with 500 bugs per millilitre 鈥� 9999 out of every 10,000 have been killed. This leads to the figure claimed on the product packaging.
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For the EN1276 test, four species of bacteria are checked, two Gram-positive, and two Gram-negative. These give a good representation of what will happen to other bugs. The species used in EN1276 are Escherichia coli, Staphylococcus aureus, Enterococcus hirae and Pseudomonas aeruginosa.
Peter Finan, Haworth, West Yorkshire
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