A try from Alex Goode and 11 points from the boot of Owen Farrell carried the London club to a 16-6 victoy at Franklin's Gardens - a result that moves them top of the Aviva Premiership table.
They managed to restrict the high-scoring Saints, who lost England duo Dylan Hartley and Courtney Lawes with knee injuries, to just two penalties in the contest.
"We pride ourselves on the defence," said Gustard.
"It is an easy way to demonstrate the unity of the squad. When things get tough we can rely on having a friend to the left and to the right to fight for each other and get that turnover when we need it.
"The scramble defence from Charlie Hodgson, somebody who was vilified while he was at Sale for not being a good defender, I think he has come to Saracens to show what a good player he is and what a good defender he is."
Northampton director of rugby Jim Mallinder was disappointed to concede the 25th-minute try to Goode after his side had made a solid start to proceedings.
"It was disappointing to concede that try when we were in attack," he said.
"We got turned over and they scored a cracking counter-attack, but I was confident we would turn it round in the second half."
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