Dental nurse 'poisoned boss's coffee with mercury' 29 April 2013 Last updated at 15:21 A dental nurse poisoned her boss's coffee with mercury after being given a final warning, a jury has been told. Ravinder Kaur, 33, of Ettrick Drive, Bedford, was reprimanded after she left a patient in a dental chair and went to lunch, Blackfriars Crown Court heard. Ms Kaur is alleged to have later placed mercury amalgam in practice manager Laura Knowles's coffee, causing her to suffer stomach cramps and sickness. She denies administering a poisonous or noxious substance with intent. Samantha Cohen, prosecuting, told the jury Ms Kaur had begun work as an assistant dental nurse at Shams Moopen Dental Practice in Shefford, Bedfordshire, in November 2011. At first she said Ms Kaur was "meticulous and professional," but her behaviour soon changed with colleagues regarding her as inflexible and "not a good team player." The jury heard that in January last year the defendant received a verbal warning for unfairly ordering her colleagues around at work. Grey sludge Two months later, Ms Kaur complained to her boss that a dentist had breached patient confidentiality and she intended to report the matter. On 13 March, Ms Kaur was summoned to a meeting to discuss her "generally poor behaviour," Ms Cohen said. The court heard a few days later Ms Kaur offered to make coffee for Ms Knowles, but did not return for 15 minutes. It was said Ms Knowles noticed her drink did not taste right and when pouring away the coffee spotted a grey sludge in the sink. Ms Cohen said she then started to feel sick and suffer stomach cramps. The prosecution said X-rays showed the presence of "speckles of metal" in Ms Knowles' lower intestine consistent with "ingested mercury." Later that day Ms Kaur was arrested. Prosecutors said swabs of sludge found in the sink matched the mercury amalgam used by the surgery to make fillings. Ms Kaur was trained in the use of mercury, said Ms Cohen, and was taught how to handle it safely and deal with spillages. Ms Cohen said the defendant claimed she had been set up by members of staff because of her reports to the authorities.